A winner of an auction will only pay the second-highest bid. This is a sealed-bid second-price auction (SBSPA) where bidders can place sealed bids (proxy) and only get to pay what the highest second bidder pledged. If a bidder loses, they get to pay nothing but all bids are final and binding on the user.
Unless you have a genuine reason for not completing your payment, if you win a bid and you fail to complete payment within 72hrs (3days), your account will be suspended and the domain will be given to the second-highest bidder. It is important to complete payment immediately to secure your wins.