If you go in and prepay for gas and you pump less gas than you have prepaid for then you will get the extra money back from the cashier.
I've prepaid more for gas than I thought I would need and then went out and pumped the gasoline.
After I pumped what my car or truck would take I would then go in and get a refund for the extra money that I paid.
I always get a receipt for the gasoline so it shows I prepaid that amount and then show the receipt again and they refund me the money back.
So if you're unsure on how much gasoline you need in your vehicle you can go in and prepay for more than you think you need.
Then go pump the gasoline and once done and you have some leftover money because you didn't pump the full amount then you can go into the store and get your money back for the rest of the money.