In August, Trump accused the company of causing "great damage to tax paying retailers," even though the internet giant began collecting sales tax on products it sells directly in April.
As with prior missives targeting the company, Trump's message appeared to concern investors. Amazon stock was positive in premarket trading on Friday and then fell as much as 0.6 per cent after Trump's post to Twitter. It was down 0.35 per cent shortly before 9 a.m.
Amazon regularly uses the US Postal Service to complete what's called the "last mile" of delivery, with letter carriers dropping off packages at some 150 million residences and businesses daily.
While full details of the agreement between Amazon and the Postal Service are unknown -- the mail service is independently operated and strikes confidential deals with retailers -- David Vernon, an analyst at Bernstein Research who tracks the shipping industry, estimated in 2015 that the USPS handled 40 per cent of Amazon's volume the previous year. He estimated at the time that Amazon pays the USPS US$2 per package, which is about half what it would pay UPS and FedEx.
The Postal Service reported a net loss of US$2.1b in the third quarter of 2017, and has US$15b in outstanding debt. The service has lost US$62b over the last decade.
Amazon has been experimenting with a new delivery service of its own that is expected to see a broader roll-out in the coming year. Under the program, Amazon would oversee the pickup of packages from warehouses of third-party merchants and the delivery to home addresses.