You can use your debit / credit card, I do it with all my PSN purchases. It basically adds the exact amount to your PSN wallet, then purchases the DLC. You don't have to do anything, the only reason I suspect that it happens like this is because every time I purchase something, I get two emails, one telling me that funds have been added to my wallet, and one to confirm the purchase.
Anywho, I've done all the bounties with all three characters now. Got one key too. I'm on nights tonight so I'll be online for a fair whack.