The fact that a merchant works with multiple CSSs won’t lead to Google charging higher CPCs.
How much an advertiser pays for an ad is based on the next best bid placed by a competitor. So if a CSS places a winning bid in the auction for one of your offers, Google calculates the cost of the click based on the next best bid for a competing merchant. Google ignores bids for any offers that also come from you. This means that whether one, three, seven, or even more different CSSs bid for you in the Shopping ad auction, the CPCs paid in the auction do not increase.
Google charges this CPC to the CSS who placed the winning bid on your behalf. Note that how the CSS ultimately invoices you for the click depends on the agreement you have with them. Some add a margin for their services, some charge fixed CPCs by category, and others charge commissions only if the click actually results in a sale. You can choose the CSSs that offer a model that is right for you.
Combining different models may allow you to benefit from different types of CSS expertise. One may have superior targeting strategies and thereby ensure higher conversion rates. Another may excel in helping you acquire new customers.