You pay only when a customer clicks on your promoted listing. The price you pay per click (your CPC) will always be no higher than your maximum bid. The exact price and whether your product is being displayed is determined in an auction where you compete with other vendors of related products for the premium placements. If you win the auction, we automatically compute your payment to be the smallest bid necessary to outbid your competition. If you face no competition, you will simply play the smallest allowed bid (the reserve price set by the marketplace).
For example, if you sell water bottles, and you would like to advertise them as a Sponsored Listing, you will be competing in an auction with other water bottle vendors. You work out how much you would be willing to pay per click to advertise your product and enter your bid. This will be your maximum bid price, you will never pay more than this price per click. If you bid the highest value, you will win the auction and pay the price the vendor below you was willing to pay.
For example, if Vendor A bid $10 per click, Vendor B bid $8, and Vendor C bid $3, Vendor A would win the auction, and their product would be listed as Sponsored. They would not pay their original bid price, but the price of the Vendor in second place, Vendor B; Vendor A would pay $8 per click.
In this way, Sponsored Listings have varying prices but you will never pay more than your bid price per click.