@WolfeReader Right: That's something I am already considering while implementing that functionality.
I'll limit the number of credits that could be purchased all at once (maybe like 1, 3, 5), and require reinputting the code as you pointed out.
Previously that's not needed but once you open up "theoretically" unlimited charges it can be abused. It's not so much protection for the consumer either, it's protection for US! If the bank/credit card issues a chargeback for a huge fraudulent purchase amount, we're on the hook for the processing fees both ways, after all.
Stripe has been pretty good about blocking sketchy cards so far... One guy from a south asian country tried to sign up using 40 different debit card numbers (they were all blocked). I kind of appreciate the clear passion for light novels the person must have had, but please, don't pay with stolen CC#s....