I went into my profile and deleted several old/canceled credit cards that were sitting there - I don't think I was accidentally choosing one of these or letting it auto-fill but who knows. I then added the new credit card that I'm trying to use. Just now I was able to use that card for a ticket. So I have to assume that one of the fields was not filling properly - I don't fat-finger that badly, usually, so I'm still blaming the website! Thanks for the suggestions.