Alternate idea: Amazon builds a central warehouse in each major metro that contains the most-ordered items for that area, has trucks that run out to UPS Stores and drop off every hour, and the customer completes the last mile and goes to pick up his package at the designated UPS Store within 2-4 hours from the time he placed the order.
I'm sure they've already considered all this kind of navel-gazing, but if customers were able to pick up within 3-4 hours of placing an order under this type of system, it'd be awesome, and I think a big win for Amazon. One of the worst things about ordering on Amazon, even as a Prime member who lives in a place where free same-day is often available, is having to wait around with no control over when the product will arrive. This would beat free same-day, which for us rarely arrives before 8:30pm no matter how early you get the order in after midnight.
Really Wal-Mart and Amazon are not so different. The difference is that Wal-Mart keeps quantities of the product within driving distance of 90% of the population, so they can have the product a half-hour after deciding they need/want it. The ability to close this gap has got to be one of the most important milestones for online retail. Ad-hoc delivery networks like UberEats are kind of already doing this. Sure would be a crazy turn of events if someone started to knock down Amazon's retail business by leveraging the latent bandwidth in the city's existing retail and crowd-sourced driver networks.