As I continue to read all this stuff about Deming and quality control, I made a realization: Amazon Prime is just a natural outcome of quality improvement.
We keep imagining (and they want us to imagine) it as something where Amazon had to spend extra money to deliver your stuff in "only" two days. But the truth is, you really should be able to get most stuff from point A to point B in two days, as long as that stuff wasn't more than two days away in the first place. And Amazon has lots of warehouses scattered around the country, so it mostly isn't very far away. (And for items that are too far, they don't offer Amazon Prime!)
We've gotten used to the Post Office delivering stuff really slowly, then charging us extra for more speed. But the speed differences are mostly artificial, due to unnecessary slowness throughout the system. A natural consequence of fixing the system is that deliveries will happen sooner with less variability, and the delivering company will save money.