President Donald Trump said Friday the U.S. Postal Service should charge Amazon.com Inc. more to deliver packages, the latest in a series of public criticisms of the online retailer and one that sent the company’s stock lower in pre-market trading.

The post office “should be charging MUCH MORE” for package delivery, the president tweeted from his Mar-a-Lago estate in Florida, where he’s been spending the holidays.

“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?” Trump told his 45 million followers.