My only explanation was that you subscribed during doc purchase or inbetween.
There is another possibility: that the address that you are giving at Paypal for paying with that credit card does not match *exactly* the invoicing address of the credit card, and thus triggers the anti-fraud address-verification...
Maybe also check that ?
But Paypal can help you in using their payment service, also without an account.
We're using Paypal, and one of the benefits is that they handle all the anti-fraud gear for us as well as the customer support in case of payment issue.
Please phone them up, they can help you, really, at least explaining why THAT credit-card won't be accepted...
From our end we don't see nor touch any credit-card number (which is another advantage for us and you too).
If things persist, please Private Message me details of Paypal reply (click the icon on bottom left of this post), and i will follow up. But you need to initiate the call with Paypal first (if it's not simply an address mismatch as said above).