The idea is interesting, but you a forgetting something. In cases of extremely low cost (like $1), sometimes the payment process itself is seen as more costly than the payment itself. In other words, people may be willing to spend $1 for an additional sentence, but would they be willing to get their wallet, pull out the card, type in the number, name, billing address and security code? I bet not.

Beyond that, using a card would force users to attach a real identity to their account, even if it's not public. A lot of people wouldn't like the idea of that.