Credit cards act as holding containers for gold. Not literally, but if you have credit in a shop and apply the card towards the shopkeeper, they can put the credit on your card, which you can then use to pay at other shops. Paying with a credit card has a 3% transaction fee, minimum 1 zorkmid. Each card also has a “credit limit” which is the maximum amount of gold you can put on it. Randomly generated cards may have some (small) preexisting balance on them, but you might get the Kops called if you try to pass it off as your own, or if you pay with a cursed credit card. A possible evil extension is that occasionally your card is invalid because the owners took all the money and fled to [other roguelike]. Priests and demon lords cannot be paid in credit. Gold that you have on a credit card doesn’t count towards score. Killing a vault guard zeroes all your credit card accounts.