If you don't want to use your own debit card, and you dont wish to use the game time cards (As Legacy players would not be getting their discount) then you could always use a prepaid Visa Card, Just load it up with how ever much money you want to and then go use it like a credit card. Put $30 on and buy crysta or something.

edit: I used this when i was 14 to play FFXI, I'd put my pocket money on every week so that i could pay for the game each month. The only details of mine it had was my name on the front and what ever money i chose to put in it.