Try deleting all of your credit card info and re-entering it. If the information is missing anything, it won't work either.
I too have the same problem "An error occurred while downloading book: unlock credit card is invalid or expired please update and try again." i have deleted the app and reinstalled. Rebooted, checked the card and bought a new book. I can not seem to find any setting on my Ipad to updated or change my credit card info. Please help