Business insider just dropped this article mentioning trump wishes to go after USPS to raise rates; to punish amazon. But as the article states he's more likely to punish all business owners. And yes, its true the USPS is losing a lot of money; along with privatized shipping agencies.... but there are a myriad of reasons and they are mostly not directly related to Amazon.
A few that I can think of, off the top of my head:
The extreme weather (fires, tornadoes, hurricanes etc.) that cripple or even destroy infrastructure to efficiently ship packages.
Various 'domestic terrorist' (I'm careful to use the word...) activities (think Austin mail bombs)
Rising fuel costs
Lower consumer confidence (due to rapidly rising costs of goods and unavailability of funds to buy them)
Consumer frustration over long shipping times (due to the above)
It doesn't look great for the consumer economy, except for a final 'mad dash' to secure various goods before various actions are taken; and with the extreme weather even those consumers will be very unhappy if and when their packages are delayed or damaged.
I dare say that Trump will eventually want to privatize the USPS, thus removing the longest standing socialist construct in the US of A; because we're making america great again!