[jaredsohn]

One thing lacking in this design is that without extra supporting content, it doesn't tell the user upfront what kinds of credit cards are supported. So if a user prefers to use American Express, they have to find the card and start entering a few digits before being told they have to try a different card (and then they may have some apprehension about if that other type of card is supported or not.)

[kenkeiter]

Great point. I'm working on coming up with something to indicate this better -- but the responsibility for that may fall to the form which contains the input, too. Not sure if it's the job of the card to indicate which types it can be.