Clear any browser cookies. If you have set it to pick up at a local store, the link will not work.
I ordered 4,000 last week for just under $11 with a 15% off coupon :) Ran into the same problem, at first the order was working, in the card, the information to place on the cards all filled out, but after I chose store delivery, it wouldn't find the offer at all. Clearing cookies, ship not pick up, and it works.
What am I going to do with 4000 business cards? Hand them out constantly. For $11, if I get even ONE new customer, I am WAY ahead :)
Timothy