PALM BEACH, Fla. - President Donald Trump is criticizing the U.S. Postal Service over how much it charges Amazon.com (AMZN) for shipping.

"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!" Mr. Trump said in a tweet on Friday.

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

The Seattle company did not immediately respond to a request for comment Friday. A spokeswoman for the Postal Service said, "We're looking into it."

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The 242-year-old postal service has lost money for 11 straight years, mostly because of pension and health care costs. Through November, the service had recorded a loss of $144 million on operating revenue of $12.2 billion.

Under a 2006 law, it must pre-fund 75 years' worth of retiree health benefits. Neither the government nor private companies are required to do that.

The post office has more than 650,000 workers, making it one of the country's largest employers.

While online shopping has led to growth in its package-delivery business, that hasn't offset declines in first-class mail. The total volume of mail, which makes up more than two-thirds of postal revenue, has dropped 27 percent over the last decade.

Starting January 21, 2018, the price of a stamp for first-class mail will rise from 49 cents to 50 cents, according to the Postal Regulatory Commission. Postcards and metered letters will increase by one cent from 34 cents to 35 cents and from 46 cents to 47 cents, respectively.

Between July and September, Amazon paid $5.4 billion in worldwide shipping costs, a 39 percent increase from the same period in the previous year. That amounts to nearly 11 percent of the $43.7 billion in total revenue it reported in that same period.

In 2014, Amazon reached a deal with the Postal Service to offer delivery on Sundays.

Federal regulators also moved recently to allow bigger jumps to stamp prices beyond the rate of inflation, which could eventually increase companies' shipping rates.

Mr. Trump often criticizes Amazon and its CEO Jeff Bezos, who also owns The Washington Post. In August, the president claimed over Twitter that the e-commerce giant is doing "great damage" to retailers and causing job losses, without specifying how.

Earlier during the summer, Mr. Trump also complained that "#AmazonWashingtonPost" doesn't pay "Internet taxes." Some speculated the tweet was prompted by a Post story that said at least some of Mr. Trump's country clubs are decorated with phony Time magazine covers featuring Donald Trump.

Shortly after the president's tweet, shares of Amazon, which had been trading higher before the opening bell, began to fade and went into negative territory. The stock was down $7.77, or 0.7 percent, to $1,178.33 as of 12:41 p.m. Eastern time. The shares have risen more than 58 percent on the year.

Amazon has taken some steps toward becoming more self-reliant in shipping. Earlier this year it announced that it would build a worldwide air cargo hub in Kentucky, about 13 miles southwest of Cincinnati.