President Donald Trump went after Amazon on Twitter yesterday, complaining that it’s paying the United States Postal Service too little to deliver packages.

Or was he complaining about the USPS? It’s hard to say. The president claimed the post office is “charging Amazon and others so little to deliver their packages.” He didn’t limit his criticism to Amazon but he did single them out, which suggests he was leveling his wrath at the online retailer. It’s not the first time that has happened.

Why is the United States Post Office, which is losing many billions of dollars a year, while charging Amazon and others so little to deliver their packages, making Amazon richer and the Post Office dumber and poorer? Should be charging MUCH MORE!

— Donald J. Trump (@realDonaldTrump) December 29, 2017

As far as the substance of Trump’s tweet, it turns out that the USPS actually turns a profit on package deliveries, while losing on rural mail deliveries. In its fiscal year 2017 report, USPS also noted that declines in first class mail and marketing mail volumes also contributed to its losses. On the other hand, revenues from its shipping and package increased by $2.1 billion, or 11.8 percent. It’s also worth noting that the USPS doesn’t operate with tax dollars, so it’s really none of the president’s concern.

Trump has also been outspoken against Amazon in the past, claiming the online retailing giant is unfairly avoiding taxes. As a candidate in February 2016, Trump threatened Amazon CEO Jeff Bezos, who also owns the Washington Post. “He wants political influence so that Amazon will benefit from it,” Trump said, of Bezos. “And believe me, if I become president, oh, do they have problems.”

It’s Bezos’ ownership of the Washington Post, a “fake news” outlet, in Trump’s parlance, that may provide a clue about the president’s animosity toward Amazon.

Amazon, for its part, defended itself against charges it pays too little to the  post office earlier this year, noting, in a statement, that the company’s partnership with USPS is reviewed annually by the Postal Regulatory Commission and that the commission “has consistently found that Amazon’s contracts with the USPS are profitable.

“Amazon has invested hundreds of millions of dollars in a network of more than 20 package sortation facilities that inject directly into the USPS last mile network bypassing most of USPS network,” the statement went on to say. “This investment resulted in more efficient processes as well as thousands of jobs and related economic benefits in local communities.”

FedEx and UPS also offer Amazon reduced rates, and the reason is simple: they are able to move Amazon’s massive volumes through their systems at a profit. No reason why the United States Postal Service shouldn’t follow suit.