While there's speculationthat Trump is using the reins of government to carry out a personal grudge because Jeff Bezos, Amazon CEO, also owns The Washington Post, the more recent obsession is based on his belief that the United States Postal Service is subsidizing Amazon's activity.
The claim is that, based on a cost-plus method of pricing, Amazon is being subsidized $1.47 per package delivered by the USPS as a last-mile carrier. With an estimated608 million boxes shipped by the online retailer in 2017, Trump is implying that Amazon has shorted the postal service by $893 million.
Considering the USPS lost $2.7 billion, this further implies that Amazon is a key reason why the USPS is struggling financially. Trump goes on to state that Amazon should fork over the entire $2.7 billion to cover the difference.