Shares of Amazon.com Inc. pared earlier gains on Tuesday after U.S. President Donald Trump repeated his unsubstantiated claim that the world’s biggest online retailer’s deliveries cost the U.S. Postal Service money.

Over the past week, Trump has stepped up his criticism of Amazon and its founder and Chief Executive Jeff Bezos, who privately owns The Washington Post, but he has offered no evidence to back up his criticisms or outlined any specific actions he planned to take.

"I am right about Amazon costing the United States Post Office massive amounts of money for being their Delivery Boy. Amazon should pay these costs (plus) and not have them bourne (sic) by the American Taxpayer. Many billions of dollars. P.O. leaders don’t have a clue (or do they?)!" Trump wrote on Twitter.

I am right about Amazon costing the United States Post Office massive amounts of money for being their Delivery Boy. Amazon should pay these costs (plus) and not have them bourne by the American Taxpayer. Many billions of dollars. P.O. leaders don’t have a clue (or do they?)!

— Donald J. Trump (@realDonaldTrump) April 3, 2018

Representatives of Amazon and USPS had no comment on Trump’s latest tweet.

Amazon's shares were up about 0.2 percent in morning trade on the Nasdaq after trading up about 1.8 percent earlier on Tuesday before Trump’s latest tweet.

The company’s shares fell more than 5 percent on Monday after Trump tweeted his opposition to Amazon’s use of the nation’s postal system and vowed unspecified change.

Last week he also attacked Amazon over taxes.