Are you a registered customer checking out while logged in, or just checking out regularly? If you're registered please PM me with your login/pass and I'll attempt an order and see what it does to me (of course I won't check out! :)
And if not, just PM me with your address that you're trying to order to and I'll test it that way...
It'll also help if you let me know what you were trying to order.
Sorry for all the troubles! and hopefully we can get this worked out soon. It must only be happening to a few people because we still seem to be getting orders like normal.