President Trump on Monday doubled down on his criticism of the U.S. Postal Service’s arrangement with Amazon, saying he would change how much the country’s largest online retailer pays in shipping fees.

“Only fools, or worse, are saying that our money losing Post Office makes money with Amazon,” he tweeted Monday morning. “THEY LOSE A FORTUNE, and this will be changed.”

The tweet marked the third time since Thursday Trump has lashed out against Amazon. The retailer’s stock was down 4.9 percent in morning trading.

Last week he attacked the retailer for paying “little or no taxes to state & local governments” and said the company uses “our Postal System as their Delivery Boy (causing tremendous loss to the U.S.)”

Two days later, he asserted the USPS loses an average of $1.50 on each Amazon delivery. “This Post Office *scam* must stop. Amazon must pay real costs (and taxes) now!” he tweeted.

Amazon collects local taxes in the 45 states that require it, although third-party sellers may have other arrangements.

Trump also incorrectly said The Washington Post is a lobbyist for the retailer. (The Post is personally owned by Jeffrey P. Bezos, the founder and chief executive of Amazon. The newspaper operates independently of Amazon.)

Amazon and USPS declined to comment on Trump’s tweet Monday morning.

The Postal Regulatory Commission, an independent federal agency, oversees the Postal Service’s pricing structure and reviews its contract with Amazon annually.

Amazon does receive a discount from the Postal Service, though the details of that arrangement have not been disclosed. An independent regulator reviews the contract every year to make sure it continues to be profitable for USPS.

Although Monday’s tweet was the first time the president implied he would try to change how much USPS charges Amazon, he has railed against its pricing structure in the past. In December, Trump attacked the company’s arrangement with the U.S. Postal Service and said the agency should raise the shipping rates it charges Amazon.

“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?” he tweeted. “Should be charging MUCH MORE!”

A fast rise in parcel deliveries — many of them from Amazon — has helped offset some of the Postal Service’s losses in recent years. In 2017, USPS delivered 589 million more packages than it did a year earlier, amounting to an 11.4 percent growth in volume and $2.1 billion increase in revenue. (Mail volume, meanwhile, decreased by about 5 billion pieces, or 3.6 percent.) Overall, the Postal Service reported a $2.7 billion loss last year on revenue of $69.6 billion.

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Trump accuses Amazon of ‘Post Office scam,’ falsely says The Post is company’s lobbyist