It's a normal feature of many market services that they serve populous communities and try to avoid serving areas with light populations. Simple economics explains it: the cost per consumer is higher in rural districts.
Whereas an organisation partly serving the common good, like USPS, is bound to provide service evenly. So it must invest in the infrastructure and labour necessary to do so. These are obligations USPS cannot avoid.
So the sorts of arrangements you are describing sound fairly logical in a mixed economy for both parties. Profit-making companies will seek to employ a supplier like USPS in rural areas. Delivering parcels for Amazon partly offsets the cost of the infrastructure and labour investment it is obliged to make for USPS.
But Trump's motives are reliably selfish and unrelated. He is targetting Amazon because Bezos also owns the Washington Post, which is hostile to him. He wants to extract a payment from Bezos. It's a shakedown of the same sort he has applied to Novartis and the other companies. Trump's methods and motives are already crystal clear.
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