How about cable companies? Some argue Amazon is subsidized in the same way, by having USPS shipping cheaply to the outskirts.
> [Amazons] rates are kept under wraps. That said, we do know the Amazon deal is at least a break-even venture.
> That’s because the 2006 Postal Accountability and Enhancement Act made it illegal for USPS to price parcel delivery below its cost.
http://www.politifact.com/truth-o-meter/statements/2018/apr/...
I live three blocks from the post office, my grandma lives 25 miles and down a dirt road from her nearest post office. It's hard to argue that my deliveries cost the same as hers, yet we pay the same price for postage.
--edit: I hold no opinion either way on the subject, just clarifying based on what I understood from reading it.
I may be incorrect, but my understanding is that while Amazon pays a discounted rate, this is the same as any other bulk shipper for USPS, and in fact, without Amazon, many routes would not be profitable for USPS without Amazon.