and as someone else said, switch to another browser.
Safari wouldn't work, changed user agent to IE11 like the other guy did, worked. (but had to go back to cart once)
GameStop, Inc. reserves the right to cancel, terminate, modify or suspend the offer for any reason without notice
Says this right in their promo discount on the checkout page lol. 100% cancelation incoming.
(or wake up and have email with "corrected" price charged to CC)