President Trump on Friday threatened to block financial help to the United States Postal Service (USPS) unless it raises package shipping prices on retail giants like Amazon.

"The postal service is a joke," Trump told reporters in the Oval Office, adding that he thinks they "should raise the price of a package by approximately four times.”

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Instead of direct aid that the struggling postal service wanted, the CARES Act Trump signed into law last month included a provision to allow the cash-strapped USPS to borrow up to $10 billion from the Treasury Department to make up for losses during the coronavirus pandemic.

Treasury Secretary Steven Mnuchin said Friday he's tying the lending plan to "postal reform." But Trump offered even more specifics: He only wants to free up the money if the Postal Service raises rates.

Trump repeated his long-held belief that Amazon and other big shipping retailers are getting too good a deal from USPS on package rates. "For some reason, they're very cozy with some of these companies," Trump said.

"If they don't raise the price, I'm not signing anything," he added.

Later on Friday afternoon, the president took to Twitter to praise postal service employees while saying the USPS has been "mismanaged" by its leaders.

"I will never let our Post Office fail," the president tweeted. "It has been mismanaged for years, especially since the advent of the internet and modern-day technology. The people that work there are great, and we’re going to keep them happy, healthy, and well!"

Trump's sharp post office critique came after he signed a $484 billion bipartisan coronavirus relief bill targeted largely at small businesses.

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Meanwhile, House Speaker Nancy Pelosi, D-Calif., accused the Trump administration of trying to use its leverage over the loan to seek to privatize the USPS. She urged Americans to speak up in favor of their mail carriers, who are especially vital for delivering medicine and communications for senior citizens, she said.

"Right now I see a very big danger for our country in the form of the Trump administration's interest in privatizing the post office," Pelosi said Friday.

She marveled at the Post Office's high approval ratings and compared them to former President Barack Obama's popularity among Democrats.

"This is really dangerous and Mnuchin at Treasury is trying to leverage the debt situation in a way that must be stopped," Pelosi said. "And the only way it will be stopped is if the American people understand what a loss it is for them."

The Postal Service repeatedly has defended its arrangement with Amazon.

The Associated Press contributed to this report.