President Donald Trump bashed Amazon on Twitter for the second time in three days, this time claiming the U.S. Postal Service loses "billions of dollars" delivering packages for the Jeff Bezos-run retailer.

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Amazon is Trump's new favorite punching bag, and that's taken a toll on the high-flying stock. At one point this month surpassed Alphabet to become the second-biggest company in the world by market value behind Apple. But the shares have since fallen back after word spread that Trump may start to take aim at the company and its billionaire CEO Bezos, who also owns The Washington Post.

Shares of Amazon lost 6 percent the last five trading days as the company came into Trump's cross hairs. Axios reported on Wednesday that Trump was looking into altering the retailer's tax treatment, in part because of anger over how Amazon has hurt the commercial real estate industry because of its negative effect on brick-and-mortar retailers.

Trump then confirmed his dislike for the retailer in a tweet Thursday. The U.S. stock market was closed on Friday for Good Friday.

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On Trump's cost claim, it was unclear what the president was directly referencing. Last year, there was a commentary in The Wall Street Journal written by a FedEx shareholder that cited Citigroup analysis. That study claimed Amazon recieved a subsidy of $1.46 from the Postal Service for each package.

The company, which declined this week to comment on the Axios report, did not immediately respond to CNBC's request for a reply.

However, some would argue Amazon has been good for the Post Office, which has suffered from poor finances for more than a decade. In fiscal year 2017, revenue from package deliveries jumped 12 percent from the previous year.

On Trump's tax claims, several states say online retailers should have to collect sales tax, even in those where the companies don't have a physical presence. The Supreme Court ruled in 1992 that states couldn't collect sales taxes gathered by mail-order catalog companies, unless the firms had a physical presence in a state. South Dakota, as well as several other states, argue that things have changed in the era of Amazon.

Indeed, Amazon does collect sales tax from consumers in 45 states and the District of Columbia. Analysts at Wall Street firm Stifel said Thursday that it appears Trump was referring to taxes on sales by third-party retailers on the website, which account for about 50 percent of volume on Amazon.

Amazon's shares are still up more than 23 percent this year and 65 percent the last 12 months.

-- Tae Kim and Michael Sheetz contributed to this report.