When you use your credit card to get gas, you are basically letting the gas station put a hold on your card while you fill up your tank with gas. This is because you may not be sure exactly how much gas you'll need. This hold could be up to $100, just to make sure you have enough on your card to pay for the gas. Once you finish your transaction, the hold will be removed, and the actual cost of the transaction will show as the final charge.