Pre-orders for Mario Kart 8 were halted on Amazon a few weeks ago, presumably because stock for the game ran out. There was no notification of when pre-orders for the game would resume, or if they would at all before the May 30 release date. Thankfully, Amazon seems to have sorted out whatever issue put a hold on their pre-orders, as they’re back open for now. If you’re looking to get your hands on this one you might want to do so now.
A few weeks ago Newegg offered a $5 gift card for those who pre-order Mario Kart 8 from them, but Amazon guarantees same day delivery of pre-order titles, so the choice is yours!