"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!," Trump said Monday on social media.
Amazon shares dropped 5.2 percent Monday.
Trump has tweeted about the e-commerce giant multiple times in recent days, spurring a big decline in the stock. Amazon shares are down more than 10 percent in the past week.
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 received a subsidy of $1.46 from the Postal Service for each package.
However, some would argue Amazon has been good for the Post Office, which has suffered from poor finances for more than a decade.
Axios reported 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 blasted Amazon over taxes Thursday saying the company pays "little or no taxes to state & local governments."
Deputy press secretary Lindsay Walters later confirmed that day Trump was referring to third-party sellers on Amazon's site when he criticized the company over taxes.
Amazon already collects state sales taxes on products it sells directly, but the company does not collect state sales taxes for its third-party platform outside a handful of states. The third-party business represents roughly half of Amazon's unit sales, according to Stifel.
On Saturday, Trump criticized Amazon's "scam" of not paying the post office adequately for its delivery services.
Sen. Marco Rubio on Monday also cited how Amazon's dominance of the retail industry may become anti-competitive in the future.
Amazon declined to comment for this story.