When using Google's own comparison engine (Google Shopping Europe - GSE) to place Shopping ads on to the Google search results page, Google apply a 'hosting' fee of up to 20% per click to do so. This fee comes from the CPC you pay making your bid to auction less competitive.

When using a CSS, the charging model is different; whether you pay a flat subscription fee or a cost-per-acquisition commission for the CSS service, you can utilise your entire CPC as your bid. This makes your bid more efficient, allowing you to enter more auctions and gain more traffic for the same cost, or to retain similar levels of activity with a reduction in CPCs driving better ROI.