Pretty sure if you pre-order something under Amazon's price guarantee and the price goes up after your order, you pay the lower cost. So if Mariah's label haven't announced a cost for the physical yet, Amazon is just covering their own backs rather than having to foot the cost if the price gets hiked up last minute.
I'm ready for Mariah to serve them bops. 
Mariah, not everyone is as rich as you, come on sis 
