A better analog to the store and Scamazon's implementation would be: You put stuff in your cart. You go through the register. You agree to "prevailing price".

A better analog to the store and Scamazon's implementation would be: You put stuff in your cart. You go through the register. You agree to "prevailing price". No prices show up because they are "calculating". You leave the store. A day later, you're hit with a surprise bill that's 10-100x more than you though. A $100 transaction ends up being $1000-$10000. There's no refunds. The dispute procedure is to beg and hope they "let you ignore it".