President Donald Trump said Friday that the U.S. Postal Service should charge Amazon.com Inc. and other companies more to deliver their packages.

“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 Friday morning. “Should be charging MUCH MORE!”

The Postal Service has said it delivers more e-commerce packages to homes than other carriers. Amazon is widely believed to be one of the Postal Service’s biggest customers for package deliveries.

A USPS spokeswoman declined to comment. Amazon didn’t immediately respond to a request for comment.

Analysts have long estimated that the Postal Service is cheaper for Amazon to use than traditional carriers such as FedEx Corp. and United Parcel Service Inc. In 2013, Amazon started taking most of its packages directly to the Postal Service.