A Comparison Shopping Service (CSS) is a website that allows users to search for and compare products and then redirect users to the merchants’ websites to make purchases.

In the European Economic Area and in Switzerland, merchants must work with one or more CSS in order to be able to place shopping ads.

If a retailer uses a different CSS than Google Shopping Europe, he receives a 20% discount on the clicks in his Google Shopping ads. The ads still run as usual on Google Shopping.