You need to support direct debit and PayPal, never mind other card types.
I currently have a problem, that probably most of your customers have every year or two. My current card is about to expire. The replacement card, which I have, is not valid until the start of the following month. I can not put my new card in, because it is not valid, but you still try to validate it, so you refuse to accept it.
The result is that I have to add my new card, first thing on the day it is valid, and hope that I get it in before you make a charge.
You should allow pre-registration of cards, and validate them when they become valid, not when they are registered.
Really though, I want to pay by direct debit, just like I do for other advertising. A subscription to PayPal would also work.
Already it is a pain having to deal with adCenter, and it is hardly generating much in the way of sales, so making it hard to pay is just going to be the nail in the coffin for some of your customers.
Jez