Advertising with Comparison Shopping Services: How Google Shopping Works via CSS

The Real Decision

The real question isn't: Should I use a CSS? Rather: Should Google Shopping represent my products or an independent CSS Partner?

An independent Google CSS Partner can display your products in Shopping ads and free product listings. Depending on the provider, you either retain full control over Google Ads and the Merchant Center or partially or fully hand over feed and campaign management to the CSS. Google differentiates between Managed Service, Self-Service, and Hybrid models for this.

A CSS is therefore not an additional advertising channel alongside Google Shopping. It is the entity through which your product data and Shopping ads are displayed on Google in the supported markets.

The Most Important Things About Advertising with CSS in Brief

• CSS stands for Comparison Shopping Service.
• Google Shopping is a CSS itself. Alternatively, you can choose an independent CSS Partner.
• Your Shopping ad still leads directly to your online shop.
• The main change in the ad is the 'By' notice, which now includes the name of the responsible CSS.
• In many cases, your existing Google Ads campaigns can remain active when switching accounts.
• A Merchant Center account can only be associated with one CSS at a time.
• You can technically work with multiple CSSs simultaneously, but this requires appropriate account structures.
• For free product listings in Europe, you can only select one CSS.
• CSS also works with automatic bidding strategies and product-feed-based Performance Max campaigns.
• The frequently mentioned 20% benefit describes a potential auction or cost effect, but not a guaranteed reduction in your actual average CPC.

What is a Comparison Shopping Service?

A Comparison Shopping Service collects product offers from various merchants and allows users to compare products, prices, and offer conditions.

For Google Shopping, the CSS takes on a second role: It can represent merchants to Google and place their products in Shopping ads or free product listings.

The CSS is the price comparison service that represents your shop in the CSS program. This can be Google Shopping or an independent provider like csspartner.io. Your product data is managed in Google Merchant Center. This includes, among other things:

• Product ID
• Product title
• Description
• Price
• Availability
• Image
• Brand
• GTIN
• Product URL
• Shipping information

The Merchant Center determines the basic suitability of your products and provides the data for Shopping ads and free product listings.

In Google Ads, you control campaigns, budgets, bidding strategies, target values, and evaluations. Depending on the chosen service model, you, your agency, or the CSS will work in this account.

Therefore, the CSS replaces neither the Merchant Center nor Google Ads. It complements the account structure as the responsible price comparison service.

Why Does the CSS Program Exist?

The origin of the CSS program lies in European competition law. In 2017, the European Commission imposed a fine of €2.42 billion on Google. According to the Commission, Google had favored its own price comparison service in search results and disadvantaged competing price comparison services.

Google then had to create a model in which other price comparison services could participate in the placement of Shopping ads. Since then, Google Shopping has acted as one CSS among several in the affected markets. Merchants can submit their product data to Google Shopping or to an independent Comparison Shopping Service.

This also explains why a Shopping ad doesn't always show 'By Google', but rather, for example, 'By csspartner.io' or the name of another price comparison service.

How does advertising through a CSS work?

The process can be divided into six steps.

1. Your product data is provided in Merchant Center – from your shop system, an XML or CSV file, a feed management solution, a direct API connection, or an automated website data source. This data determines which products and search queries your ads are eligible for.

2. The Merchant Center is associated with a CSS. When switching, an existing account can be transferred from one CSS to another. The new CSS initiates the transfer, and a Merchant Center administrator confirms or rejects it.

3. The Merchant Center is linked to Google Ads. This connection allows product data to be used in Shopping or Performance Max campaigns. A CSS change does not automatically mean you have to change your existing Google Ads account.

4. Your products participate in the ad auction. Google checks product relevance, product data, bid or bidding strategy, expected ad quality, competition, user context, and campaign and budget settings, among other things. The selected CSS submits the bid for the merchant it represents in the auction.

5. The Shopping ad is displayed – typically with a product image, product title, price, shop name, possibly ratings, and the name of the CSS. Clicking the CSS notice leads to the price comparison service's website.

6. Clicking on the product leads to your shop. A normal click on the Shopping ad does not first lead to the CSS website, but directly to the merchant's product page. All CSSs and merchants are subject to the same basic Shopping policies and requirements for product data and landing pages.

What changes with an independent CSS Partner?

A change alters less than many merchants initially expect.

The extent of operational change depends on the service model. With a lean self-service connection, the change may essentially be limited to the CSS association. In a full-service model, on the other hand, the provider can fully take over Merchant Center, feed, Google Ads, and campaign management.

Planned expansion of the CSS program

Important: The active CSS markets are not exclusively EU member states. Switzerland, Norway, and the United Kingdom are also included. Therefore, '21 European CSS markets' is more precise than '21 EU markets'.

Google has announced an expansion to other countries in the European Economic Area. Bulgaria, Cyprus, Croatia, Slovenia, Estonia, Latvia, Liechtenstein, Lithuania, Luxembourg, and Malta are mentioned.

According to the information published by Google, product data can already be prepared, but ad serving in these markets is only expected to begin after separate approval. Therefore, you should always check the current list of countries before starting a campaign.

How does the often-cited 20% advantage work?

The so-called CSS advantage is often described with phrases like '20% less CPC', '20% more bidding power', or '25% higher effective bid'. These statements do not all refer to the same basis of calculation.

Google explains in its own Shopping Solutions FAQ that Google Shopping applies a fixed percentage surcharge to the Cost-per-Click determined in the auction. In practical calculation, this margin is deducted from the bid before the auction and then taken into account again.

Independent CSS Partners set their own pricing model. They cannot retain an auction margin, but may charge a monthly basic fee, demand a percentage CPC surcharge, bill by revenue or sales, or sell additional service packages. csspartner.io states that it passes on the full bid to the auction and passes on the resulting effect as approximately 20% lower CPC or additional bidding power.

For illustration, let's assume the retained margin is exactly 20%:

The auction bid of the independent CSS in this example is 20 cents higher than the original one-euro bid. Compared to the 0.80 Euro that previously went into the auction, 1.00 Euro is 25% more: 0.20 € ÷ 0.80 € = 25%.

This leads to the different statements: 20% less bid required to mathematically get back to 0.80 Euro – or 25% more effective bidding power if the original bid of 1.00 Euro remains unchanged. Both statements can be mathematically correct. They simply refer to different starting values.

Does this automatically mean 20% lower click prices?

No. The CSS effect is an auction advantage, but not a guarantee of a specific campaign outcome. The actual average Cost-per-Click (CPC) still depends on competition, product relevance, bidding strategy, target ROAS or CPA, conversion data, budget, search demand, season, prices, shipping costs, and product feed quality.

For example, with automated bidding strategies, Google may use the additional bidding opportunity to access more valuable auctions. In this case, the average CPC does not necessarily decrease proportionally.

More precisely, an independent CSS can improve your effective position in the Shopping auction. Whether this results in lower CPCs, more reach, or a combination of both depends on your campaign goals and the auction situation.

Three strategies after switching to a CSS Partner

Strategy 1: Increase reach and revenue. Initially, you keep budgets, bids, and target values unchanged. Due to the higher effective auction bid, your products can become competitive in more auctions – potential consequences include more impressions, a higher share of potential impressions, more clicks, higher conversion volume, and more revenue. This strategy is suitable if your campaigns are already profitable.

Strategy 2: Improve efficiency and profitability. You gradually reduce your bids or tighten your efficiency targets. For manual CPC bids, a reduction can be tested directly. For target ROAS, Maximize Conversion Value, or Performance Max, you should proceed more cautiously – an abrupt change can affect the campaign more significantly than the actual CSS switch. Monitor average CPC, cost per conversion, conversion value, ROAS, impression share, and lost impression share.

Strategy 3: Measure the effect in a controlled manner. For most merchants, this is the most sensible approach. Keep the essential campaign settings stable initially and document the development. Do not simultaneously change CSS, product feed, target ROAS, budget, and campaign structure – otherwise, it will be difficult to determine which measure was responsible for the change.

Full Service, Self Service, or Hybrid?

CSS Partners can offer very different services. Google distinguishes three basic models.

In the Full-Service model, the CSS often creates or manages the Merchant Center, Google Ads account, product feed, campaigns, bids, and reporting. Billing can be a monthly agency fee, a fixed click price, a percentage of ad spend, revenue share, or performance-based commission. The advantage lies in low internal effort, the disadvantage in greater dependence on the provider. Therefore, check carefully who owns the accounts.

In the Self-Service model, you manage product data and campaigns yourself or through your existing agency. The CSS primarily handles CSS assignment, setup, migration, technical support, and, if applicable, additional reports or tools. You shouldn't pay a second campaign agency if your team already reliably handles this task.

The Hybrid model divides responsibility: the CSS optimizes the feed and you manage the campaigns, or vice versa. This model is flexible but requires clear responsibilities. Document in writing who is responsible for feed errors, disapprovals, tracking, budgets, and campaign changes.

Does a CSS Partner work with Performance Max?

Yes. Product feed-based Performance Max campaigns use product data from Google Merchant Center. If the Merchant Center is assigned to a specific CSS, the Shopping activities used from it belong to that CSS. Google fundamentally provides all CSSs with functions such as conversion reporting and automated bidding strategies.

However, Performance Max is broader than a classic Shopping campaign. Campaigns can appear in Google Search, the Shopping tab, Google Images, YouTube, the Display Network, Gmail, Demand Gen, and Google Maps, among other places.

The CSS primarily affects product-related delivery via the connected Merchant Center feed. Therefore, you should not automatically attribute every change in the entire Performance Max campaign to the CSS. Consider product performance separately, check Shopping and Search placements, note changes in channel distribution, and compare conversion value instead of just click prices.

By default, a CSS can place products in the Shopping unit of the general Google search results. Some CSSs have additional approvals for other surfaces such as the Shopping tab, YouTube, the Display Network, or dynamic remarketing. Before switching, you should check whether the new provider supports the surfaces you need.

How do free product listings work with a CSS?

In addition to paid Shopping ads, Comparison Shopping Services (CSSs) can also represent products in free product listings. However, there's an important restriction: for free product listings in Europe, only one CSS can be selected per merchant domain.

This selection is made in the CSS dashboard of Merchant Center. If the CSS chosen for free product listings doesn't match the Merchant Center account used, products may be disapproved for this destination.

When switching, you should therefore not only review your paid campaigns but also check the selected CSS for free listings, the product's destination programs, approvals in Merchant Center, country assignments, and existing diagnostics and disapprovals.

Can you use multiple CSSs simultaneously?

Yes. Google generally allows merchants to advertise their products through multiple CSSs simultaneously. However, a Merchant Center account can only be associated with one CSS at a time. For parallel use, different Merchant Center structures or separate accounts are therefore used.

Will you be bidding against yourself? No. Google states that a merchant will not be second-priced against themselves in the auction. If CSS A places a bid for your shop and CSS B places another bid for the same shop, the next highest bid from another merchant will be considered for the click price to be paid.

Google also tries to show identical offers for the same product and merchant only once in a Shopping unit. In rare cases, two offers may appear if they are not reliably identified as identical. Different products from the same merchant, however, can certainly be displayed simultaneously via various CSSs.

Parallel use can be interesting if different CSSs cover different countries, one provider manages the feed and another the campaigns, or a large organization separates different brands. For many small and medium-sized merchants, a single reliable CSS is easier to manage. A multi-CSS setup should serve a specific purpose and not just be implemented because it's technically possible.

How can you switch your CSS Partner?

Switching doesn't necessarily mean you have to rebuild your campaigns.

Step 1: Review your existing setup. Document your Merchant Center ID, Google Ads account, linked accounts, target countries, product data sources, free product listings, dynamic remarketing, local inventory ads, additional Google surfaces, and current users and access rights.

Step 2: Select a new CSS. Clarify in advance: country coverage, service model, pricing, contract duration, support, account access, billing, migration assistance, ad serving outside of general search, and how free product listings are handled.

Step 3: Initiate the switch. The new CSS will initiate the Merchant Center account transfer. A Merchant Center administrator will then have the option to approve or reject the change.

Step 4: Check your Google Ads link. Your Google Ads account usually doesn't need to be transferred during a normal CSS switch. Nevertheless, check that the Merchant Center link is active, all campaigns are accessing the correct product data, no products have been unexpectedly excluded, conversion tracking is still working, and automatic bidding strategies are receiving data.

Step 5: Monitor ad serving. Check product status, impressions, clicks, costs, average CPC, conversion volume, conversion value, ROAS, free product listings, target countries, remarketing, and other surfaces. Google points out that after a switch, ad serving outside of general search or in countries not part of the CSS program may cease if the new CSS does not support these areas.

What does a CSS Partner cost?

CSS Partners use different pricing models. With a monthly fixed fee, you pay a set amount regardless of your Shopping budget – easy to calculate and relatively cheaper with an increasing ad budget. With a percentage CPC markup, the CSS charges a surcharge on click costs; this seems cheap for small budgets but automatically becomes more expensive as your ad spend increases.

When based on a percentage of ad spend, the fee is oriented towards the managed Shopping or Google Ads budget – clarify whether the percentage refers only to Shopping or to all campaign types. For revenue or performance-based models, attribution models, returns, cancellations, taxes, and shipping costs must be clearly defined. A full-service package combines CSS, feed management, Google Ads management, and reporting, making direct price comparison difficult.

Don't just use the publicly advertised starting price. Total annual costs = CSS fee + setup + countries + accounts + tools + campaign management + support + internal effort. A free CSS might require paid agency support or software. The crucial point isn't whether the individual CSS line item is free, but what the complete solution costs.

How do you find the right CSS Partner?

The auction mechanism is only one part of the decision. In day-to-day operations, providers primarily differentiate themselves through service, technology, costs, and contract terms.

Google allows you to search for CSS Partners based on various criteria. According to Google, Premium Partners have additional technical certifications and offer support for conversion tracking, among other things. However, Premium status does not replace your own comparison.

Ten questions for the provider discussion: What are the one-time and monthly costs? What services are included in the price? For which countries is the CSS enabled? Which Google surfaces are supported? Will my existing Merchant Center be retained? Will my existing Google Ads account be retained? Who owns the accounts and administrative rights? How do termination and subsequent switching work? What support is included? Can agencies offer the service as a White Label?

Please get all answers in writing. This helps you avoid later misunderstandings about responsibilities or additional costs.

The most common mistakes when using CSS

• Mistake 1: Expecting 20% lower CPCs as a guarantee. The auction advantage can manifest as a lower CPC, additional reach, or a mix of both.
• Mistake 2: Lowering all bids immediately after switching. If you change CSS, budget, target ROAS, and campaign structure simultaneously, you can no longer isolate the effect.
• Mistake 3: Viewing CSS as a substitute for feed optimization. Precise titles, correct prices, current availability, suitable images, unique product identifiers, high-quality product pages, and competitive shipping conditions remain crucial.
• Mistake 4: Only comparing the cheapest monthly fee. A low price helps little if countries are missing, support is unavailable, or account access remains unclear.
• Mistake 5: Forgetting free product listings. The CSS for paid ads and the selection for free product listings must be properly configured.
• Mistake 6: Ignoring markets outside the CSS program. If you also sell in the US, Canada, or Australia, your account model must also cover these markets.
• Mistake 7: Evaluating campaign performance based only on CPC. Orders, conversion value, contribution margin, cost per order, new customer share, returns, ROAS, and profitability are decisive.

How do you measure the success of a CSS switch?

Before switching, document a reliable baseline. Record at least impressions, clicks, click-through rate, average CPC, costs, conversions, cost per conversion, conversion value, ROAS, impression share, impression loss due to ad rank, and the number of active and disapproved products. For Performance Max, you should also check product groups, search categories, and available channel information.

Do not compare a normal sales week with Black Friday, Christmas, a sale, a product range change, a price increase, or a seasonal peak in demand. A before-and-after comparison is only useful if the underlying conditions have not fundamentally changed.

Also note: Date of CSS switch, budget changes, new target values, feed adjustments, price promotions, product range changes, tracking updates, new campaigns, and changes to conversion goals. This will help you better understand later performance differences.

Who benefits from an independent CSS Partner?

A CSS Partner is particularly interesting if you are already actively using Google Shopping, employing Performance Max with a Merchant Center feed, your Shopping budget significantly exceeds the CSS fee, your campaigns are generally profitable, you are looking for additional reach or efficiency, you want to manage your campaigns yourself, you already work with a Google Ads agency, you sell in multiple European markets, or you want to offer CSS to your clients as an agency.

The potential benefit is smaller if you are not yet running Shopping campaigns, your advertising budget is very small, your product feed contains many errors, conversion tracking is not reliably working, your products are not approved on Google, you only advertise outside of CSS markets, or your shop barely converts buying-intent traffic.

CSS optimizes the auction's framework conditions. It does not replace a functional e-commerce and campaign strategy.

Conclusion: CSS Is More Than Just Another Name Under the Ad

A Comparison Shopping Service connects your product feed with Shopping ad placements in the supported European markets. The choice of partner influences the commercial auction model, your ongoing CSS costs, support and service, account access, country coverage, available interfaces, feed and reporting functionalities, as well as the effort involved in setup and switching.

The frequently cited 20% advantage is an important reason to consider an independent CSS. However, you shouldn't mistake it for a guaranteed reduction in average CPC.

A good transition follows a controlled approach: document initial performance, select a suitable CSS, check accounts and access, execute the switch cleanly, initially keep campaign settings stable, compare reach, costs, and conversion value, and then optimize bids and target values.

Switch to an Independent Google CSS with csspartner.io

csspartner.io offers CSS connectivity as a flat rate: no setup fees, no click commission, and no minimum contract term. The migration is handled for you, while you retain full control over campaigns and budget.

This allows you to leverage the CSS advantage without having to hand over your existing Google Ads strategy to a new campaign manager.