NurPhoto

For the third time in five mornings, President Trump is attacking Amazon's business practices via Twitter.

On Saturday, two days after his initial jab, Trump sent off a one-two punch, calling Amazon's use of the US Postal Service a "scam" and swiping at the company's lobbying efforts.

And then on Monday, yet again -- another poke, repeating the argument that the Postal Service loses money shipping for Amazon. "This will be changed," Trump said.

Only fools, or worse, are saying that our money losing Post Office makes money with Amazon. 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! — Donald J. Trump (@realDonaldTrump) April 2, 2018

Amazon's shares, along with the broader market, fell Monday morning, with the e-commerce company's stock down about 5 percent.

Amazon and its CEO, Jeff Bezos, have been frequent targets of Trump's attacks, with the president arguing that the company isn't paying its fair share in state and local taxes and is gaining an unfair advantage by using the US Postal Service to deliver its packages. Trump is widely seen as targeting the company because Bezos owns The Washington Post, which has reported critically on the president.

Trump on Saturday alleged that the US Postal Service is losing an average of $1.50 for every package it delivers for Amazon, amounting to billions of dollars. He may have been referring to a widely circulated Citigroup report from last year that said the Postal Service would need to charge 50 percent more for packages to cover all its costs. "This Post Office scam must stop," Trump tweeted.

While we are on the subject, it is reported that the U.S. Post Office will lose $1.50 on average for each package it delivers for Amazon. That amounts to Billions of Dollars. The Failing N.Y. Times reports that “the size of the company’s lobbying staff has ballooned,” and that... — Donald J. Trump (@realDonaldTrump) March 31, 2018

...does not include the Fake Washington Post, which is used as a “lobbyist” and should so REGISTER. If the P.O. “increased its parcel rates, Amazon’s shipping costs would rise by $2.6 Billion.” This Post Office scam must stop. Amazon must pay real costs (and taxes) now! — Donald J. Trump (@realDonaldTrump) March 31, 2018

Amazon, along with other e-commerce companies, routinely does work with the Postal Service, and the arrangement saves it money. Some observers argue this amounts to a government subsidy for Amazon and other shippers. But others say Amazon, one of the Postal Service's biggest customers, is a benefit for the agency.

Amazon uses the Postal Service for last-mile deliveries, called "postal injections." It sends packages to local post offices, which can deliver items more cheaply because the Postal Service sends mail carriers to every address already. Bigger companies are usually the only ones with the logistics capabilities to do postal injections.

Trump on Saturday also mentioned that Amazon is amping up its lobbying expenditures, arguing that The Washington Post is part of that lobbying operation and should register as such. The Post is a news publication and isn't directly involved in lobbying for Amazon.

The Post didn't respond to a request for comment, but in a story about the tweets Saturday, it pointed to a piece it ran Friday about the legal inquiries into the Trump Organization's finances.

Despite continued saber-rattling against Amazon, Trump has yet to take direct action against the company, and his staff said last week that the White House has no current plans to do so. Meanwhile, the general public continues to view Amazon as one of the most reputable companies in the US. Trump's previous assertions that Amazon pays little to nothing in state and local taxes are inaccurate.

Amazon declined to comment for this story. The White House didn't respond to a request for comment.

First published March 31, 7:04 a.m. PT.

Update, 7:45 a.m. PT: Adds more background.

Update, April 2 at 8:28 a.m.: President Trump tweeted about Amazon again Monday, repeating his argument that the Postal Service loses money shipping for Amazon. "This will be changed," he said. Amazon's shares, along with the broader market, fell Monday morning, with the e-commerce company's stock down about 5 percent.

The Smartest Stuff: Innovators are thinking up new ways to make you, and the things around you, smarter.

Blockchain Decoded: CNET looks at the tech powering bitcoin -- and soon, too, a myriad of services that will change your life.