Their delivery schedules and prices are crazy in how they come and go. I've often wondered if their warehouse is so gigantic that they 'lose' things only to rediscover them later after clearing out more stock only to realize they missed something. I've gotten 9mm from them four separate occasions during the past 3 months they they must have purchased years ago as it is like 2006-ish pricing.
Bought more. This deal is like being offered to super-size my fries for 39 cents. I just can't refuse the price. They'll get me every time.
Worst case: you decide you don't want them and sell for double what you paid.
[Update] one more consideration: Magpul is about to shift all orders to Colorado residents and then fold up shop to move. There is going to be a mag shortage for a LONG TIME.