I've been getting this error as well for the past few days when trying to purchase the Blast Off Bag. When I added the Tall Cup to my cart and proceeded through the ordering process, the final order only contained the cup and not the bag!
Very frustrating when I'm trying to give a company money and the ordering system doesn't work properly.
What internet browser are you using? It would also help if you could let us know what operating system you are using too...