Shares of Amazon.com Inc fell 4 percent on Monday after U.S. President Donald Trump again attacked the online retailer over the pricing of its deliveries through the United States Postal Service and promised unspecified changes.

"Only fools, or worse, are saying that our money losing Post Office makes money with Amazon," Trump tweeted.

"They lose a fortune, and this will be changed. Also, our fully tax paying retailers are closing stores all over the country...not a level playing field!"

Shares of the company were down 3.9 percent at $1,389.50.

Trump has been vocal about his opposition to Amazon's use of the postal service and Monday's tweet adds to investor worries that the company could see more regulation.

Amazon did not immediately respond to requests for a comment.

Details of Amazon's payments to the U.S. Postal Service (USPS) are not publicly known, but some Wall Street analysts have estimated it pays the postal service roughly half what it would to United Parcel Service Inc or FedEx Corp to deliver a package.

"President Trump's comments are consistent with industry sources we have spoken to in the shipping industry, who often label Amazon's deal with the USPS as a sweetheart deal," DA Davidson analyst Tom Forte wrote in a note.

"An argument, however, could be made that the USPS was losing billions before it expanded its service offerings for Amazon and would, still, likely lose billions if Amazon discontinued its use of the USPS tomorrow," Forte said.

Morgan Stanley analyst Ravi Shanker wrote in a note that the brokerage believes USPS has gained share from Amazon, which is helping USPS stay afloat.

Trump last Thursday accused Amazon of not paying enough tax, making the postal system lose money and putting small retailers out of business.

But he offered no evidence to back up his criticisms and did not suggest any actions he would take.

Amazon shares have gained nearly 20 percent this year giving the company a market value of about $700 billion (497.87 billion pounds).