I was trying to change my payment info to a new credit card so I wouldn't have to use my old one - it wouldn't take it, it kept saying the address didn't match what was on file at the new bank. NOTHING has changed in my address. I even checked on the bank website to make sure every letter, number, space, period and blank was correct. THEN your system wouldn't let me go back to my OLD card until about the 5th time of starting over - it was frustrating!
Also, I would like to suggest you have something in place that lets us know "You've bought that book before (on whatever date), would you still like to make this purchase?" Because I bought the same book twice last year and didn't know it. Never found the first one that was sent to me...