Blizzard always claims it's your bank, even though it never actually is.
Easiest and much cheaper way to upgrade accounts is to walk to your local game store, purchase retails, enter cd keys to battle.net, enjoy
I remember this now... I did go out and buy boxes and upgrade that way. My bank shut off my card but if I went to store I could by 5 copies at once and no problem, even buy time cards as well.