First, why is the postal service competing with private enterprise at all?
Second, Amazon didn't force the postal service to price its services the way they did. If it is improperly priced that is entirely on the postal service and those who govern it.
Third, this only makes Trump look vindictive, which obviously he is, and upsets people who use Amazon Prime which are many. Stupid thing to do.
Last, Trump should have simply worked to fix the postal service and not target their customers as the fall guy. That wouldn't have angered Amazon customers while at the same time fixing a problem that needs fixed.