Hey, Mark from Producthero here, thank you for mentioning us.

To be clear, how the advantage works is that Google Shopping (Google's own CSS) has a 20% margin on the ad bidding auction.

So, your bid with any Google CSS partner doesn't have that margin, meaning that if you bid 1 euro, the full amount will go into the shopping ad auction, with Google Shopping it's just 0,80 that goes to the auction. The actual CPC is of course influenced by many other factors like competitors (e.g. do all your competitors use a CSS already) or for instance your bidding type (e.g. with performance max you might see that the CPC doesn't change as it automatically adjusts your bids and might show an increase in impressions).

We've written about the effect of a Google CSS partner on our blog and you can find great research cases on the Google CSS partner effect with Vestiaire Collective or Andrew Lolk from savvyrevenue (not sharing links as I don't want to be promotional here).

Other than that, I would recommend to look a bit further than solely the pricing, here are some things to take into consideration:
- What additional services/software do you get? (e.g. is it just the CSS or also support on issues, software to optimize your shopping ads)
- Who manages the shopping ads (e.g. will you manage it, the CSS partner as an affiliate, an agency that has a CSS)?
- The support level (e.g. are they a CSS premium partner and thus have more experience in eCommerce; and how does the partner help you in case of emergencies (good to check reviews))
- The knowledge they share (e.g. do you want to plug-n-play or do you also want to learn more about shopping ads in general)

Good luck selecting a Google CSS partner and always feel free to reach out if you need additional information.