Will I end up paying more for Shopping ads if more than one CSS advertises on my behalf?

A merchant will never be second-priced against itself in the auction for any offer, and that rule holds irrespective of the number, or identity, of the CSSs used by the merchant. Therefore, if two CSSs place bids on behalf of the same merchant, the winning offer and the price paid by the winning CSS will be the same as if those bids had been placed by one and the same CSS.

The fact that several CSSs place ads on your behalf won’t mean your offers are being repeated or that Google will charge more for a click on any of them.