(USA Today) - President Trump ramped up his criticism of Amazon Friday, tweeting that the e-commerce giant should be socked with sharply higher shipping charges by the U.S. Postal Service.

"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!" Trump said in the early-morning tweet during a Christmas season vacation at his Mar-a-Lago estate in Florida.

The tweet at least temporarily shifted Amazon shares from positive to negative. The company's stock was fractionally lower at $1,1779 a share in afternoon trading.

Representatives of Amazon and the postal service did not immediately respond to emails seeking comment.

After reporting a surplus in 2006, the postal service has reported net losses during each of the 11 succeeding fiscal years. The 2017 net loss totaled $2.7 billion, the postal service reported in November. Most of the losses resulted from changes in interest rates, "outside of management's control" that reduced workers' compensation costs.

Earlier in December, the Postal Regulatory Authority moved to stem the red ink by allowing the postal service to raise the cost of a first-class stamp, currently 49 cents, by 2% above the rate of inflation. The move eventually is expected to raise shipping costs for Amazon and other companies, as well as for magazines, prescriptions, and other mail.

In contrast, Seattle-based Amazon has become a flourishing retail giant, shipping a wide range of products to consumers who increasingly prefer to do their shopping online. Amazon's stock for the first time topped $1,000 a share in April, and have continued to climb.

Trump has directed a series of critical tweets at Amazon, whose founder and CEO, Jeff Bezos, also owns The Washington Post — the national news organization that has broken many news stories that spotlighted shortcomings involving the president and his White House administration.

For instance, Trump in July tweeted that the "Amazon Washington Post" fabricated information about his decision to end U.S. payments to rebels fighting the forces of Syrian President Bashar Assad.

Shipping expenses represent a major expense for Amazon. Although the company sends some deliveries through the U.S. Postal Service, it also uses UPS and Federal Express for shipping.

FedEx and UPS shares fell in October amid news reports that Amazon was testing its own delivery system to whisk packages from the company's warehouses to consumers' homes.