Zombies!!! is $12 shipped from Amazon third party seller QB's & Pitchers, no Prime membership required. This is very close to the lowest price ever according to camelcamelcamel.com and is lower than dedicated game stores by about $7 before shipping.
Deals from the Amazon third party seller QB's & Pitchers have been popping up all over Hot Deals lately. It seems like their full games list gets buried in individual game threads so I'll try to create new threads any time I notice something that's a particular deal. They've been steadily dropping prices over the last few weeks and continue to restock every day or two so even if something shows out of stock now, don't give up and check back tomorrow. Most notably, many of their games have free shipping that does not require a Prime membership.
Some deals I've noticed so far that are the lowest available offer from all sellers. All prices include shipping and don't require a Prime membership. It's still worth browsing all their games though since I only covered games I recognized as deals.