I’m pretty sure the issue is that Amazon has their own fleet of trucks where costs are low ... like in major cities.
Then they can dump off the high-priced areas, like rural, to the USPS.
It’s just rational business on their part, I must say.
The Post Office has a long history of charging the same price for deliveries regardless of location. If they keep this practice, but increase prices as POTUS is demanding, then Amazon and others will expand their own fleets removing additional “profitable” areas from the USPS.
Possibly the high costs of rural areas will be mitigated when packages and letters are delivered by drones.
Another point is a lot of stuff is mailed to the US from foreign countries and then the USPS delivers it, having received no revenue at all from those countries. A lot of Amazon stuff too. Basically the US subsidizes other countries’ postal systems. A friend tole me she mailed something to a friend in Canada and it cost aroung $12, something small. Friend mailed back a similar item and it cost her $1-2 to send from Canada to the US.