You can magically charge up a credit card, which when you take it into a shop makes it good for a certain amount of store credit. Alternatively, applying a credit card with charges just spits out some zorkmids and uses up a charge.

Leprechauns have a tiny chance, less than 1%, of dropping a credit card on death (or generating with one). Such a card will always be a “Yendorian Express - Gold Card” or “Shamrock Card” when read. They also are capable of stealing any credit card that says either of these things.

Attempting to pick a lock with a credit card may result in it slipping through the crack (ending up either inside the box or on the other side of the door). The chance for this is getting a 1 on a rnd(Dex*2) roll (with an uncursed card; cursed cards will always slip through and blessed ones will never slip through). If fumbling, use Dex/4 instead of Dex*2.

Every lock in the game has a deterministic chance (based on object ID if a box and xyz coordinates if a door) of being unopenable by a credit card. Attempting gives the message “It doesn’t look like this lock can be opened with a credit card.”

• Boxes have a constant 60% chance of being like this, and doors are based on depth, starting at 10% or so on level 1 and increasing to 80% by level 50.
• Possibly extend this behavior to locks that can’t be opened by a key either, requiring either a lock pick or magic.

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.

Leprechauns steal credit cards as well as money.

80% of locks on containers, determined by the object id of those containers, can’t be opened by a credit card because they’re a latch or padlock or something that can’t be jimmied open. Perhaps Tourists get a bonus that allows them to open more locks.