President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE is not backing down on his argument that Amazon financially hurts the U.S. Postal Service (USPS), tweeting on Tuesday that the mega-retailer costs USPS “massive amounts of money.”

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“I am right about Amazon costing the United States Post Office massive amounts of money for being their Delivery Boy. Amazon should pay these costs (plus) and not have them bourne [sic] by the American Taxpayer. Many billions of dollars. P.O. leaders don’t have a clue (or do they?)!” Trump tweeted.

I am right about Amazon costing the United States Post Office massive amounts of money for being their Delivery Boy. Amazon should pay these costs (plus) and not have them bourne by the American Taxpayer. Many billions of dollars. P.O. leaders don’t have a clue (or do they?)! — Donald J. Trump (@realDonaldTrump) April 3, 2018

The president’s strike at Amazon on Tuesday follows a series of tweets in which he criticized the company’s shipping arrangements with USPS.

"Only fools, or worse, are saying that our money losing Post Office makes money with Amazon," the president tweeted Monday. "THEY LOSE A FORTUNE, and this will be changed. Also, our fully tax paying retailers are closing stores all over the country...not a level playing field!"

The USPS does give Amazon a shipping discount because of the volume of packages it ships and has a deal with the USPS to ship packages on Sundays, but details of the arrangement have not been made public.

The accusation that Amazon is hurting the USPS has been disputed, with some fact-checkers saying that while the Postal Service is having problems, it is not because of Amazon.

Amazon owner Jeff Bezos has long been a target of Trump.

Bezos also owns The Washington Post, and Trump on Saturday accused him of using the newspaper as a "lobbyist" for Amazon. He has also often accused the Post of being "fake news."

The Post and Amazon are two separate businesses and operate independently of one another.