President Donald Trump continued challenging online retailer Amazon.com for using the United States Post Office at a discounted shipping rate.

“I am right about Amazon costing the United States Post Office massive amounts of money for being their Delivery Boy,” he wrote on Twitter.

Amazon frequently uses the USPS Parcel Select service to deliver packages, which allows them to use the service at a discounted rate.

“Amazon should pay these costs (plus) and not have them bourne (sic) by the American Taxpayer,” Trump wrote. “Many billions of dollars. P.O. leaders don’t have a clue (or do they?)!”

From 2007-2017, the Post Office has lost $65.1 billion.

Amazon defenders have urged the president to stop attacking the behemoth company led by multibillionaire CEO Jeff Bezos, but to no avail. Trump wants the Post Office to charge more for package delivery. He is also concerned about the Pentagon contract with Amazon for cloud computing.

Trump has repeatedly attacked Amazon for damaging retail brick-and-mortar businesses, arguing that they do not compete on a level playing field.

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