After an hour of frustration with trying to figure out what's wrong, enabling scripts globally in noscript in firefox finally did it for me. When you submit the CC info you're entering it into SE's site. The site then passes it to the CC processor when you click the button. So there's no way to allow scripts from that site before submitting (it doesn't show up in the list at that point), and no indication that that's what's wrong when it fails. So I guess for me it was just simple user error.