I believe it has to be a credit or debit card. Not sure if you're aware but the card is just held on file. When you use your MB you are charging to the room account and then that is applied to the card on file. The individual charges don't hit your card. You can always go to the front desk every couple of days to pay off the balance on your room account with some other method (cash/GC) so that the charges are not applied to the credit/debit card on file.