This is hilarious for anyone who has been in prison. Our commissary in Wisconsin was similar, except we called it "canteen", and we didn't have MP3 players. Well, a few people did but they were smuggled in and very expensive. Our currency standards were ramen noodles and stamped envelopes. The administration tried various things, stamping our inmate numbers on the envelopes, etc, but eventually gave up. In maximum security, supposedly the highest level, cash and drugs also regularly changed hands. Poker games were played for large amounts of money, fantasy football, inmate run stores, inmate crafts, prison was a free economy. It's worth noting that the guards didn't generally side with admins on the issue of preventing trade. They didn't want to write conduct reports and deal with the paperwork. So after canteen was delivered, they would usually give us a grace period to settle our debts.