For those of you who had 4 successful coupon uses, and then got your purchase/bookshelves blocked during the actual checkout on a 5th try (rather than while trying to enter the coupon code again or however it works):
Might it be possible that perhaps it's your CC that blocked and failed the purchases due to some sort of automated repeated small charge amounts triggering a fraud warning at your CC company?
Because it sounds kind of like
this situation in General Discussions where someone tried to buy an unusually large amount via Books on Board, and got the payment held for examination or whatever, and his bookshelf locked until stuff was cleared up.