They're usually card-based. You insert (or in past decades swipe) your card, a hold gets put on your account, you pump the gas, and the measured amount gets rem

They're usually card-based. You insert (or in past decades swipe) your card, a hold gets put on your account, you pump the gas, and the measured amount gets removed from your account & the hold removed. If you don't have enough in your account for the hold, the transaction declines. It's effectively pre-pay, even though the transaction settlement doesn't happen until after the gas is pumped.