It is really a maximum limit. The amount you actually pay will be reduced if possible. This type of mail-bid sale is intended to reproduce the conditions of the classic public auction except for having no floor bidders. There may or may not be a buyer's fee. Look at the auction terms and adust your bid accordingly.
Most coins do sell pretty close to the estimated value.
Most major coin firms are proud that their coins actually sell, on average, for about the estimates, or even above. It is very important to have some idea of the value of a coin before bidding on it. Mail-bid auctions with bids not reduced . In these auctions, if your bid is high you pay what you bid. In these, you pay what you bid, even if no second bidder pushes you up to that amount.