President Donald Trump is again taking aim at Amazon for "using" the U.S. Postal Service as its "delivery boy," raising the possibility of antitrust claims against the company.

Trump railed against the e-retailer and The Washington Post — owned by Amazon CEO Jeff Bezos — in a series of tweets Monday morning, claiming the newspaper has "gone crazy" against Trump in the months since Amazon "lost the Internet Tax Case in the U.S. Supreme Court."

Shares of Amazon fell nearly 2 percent in early trading Monday before paring half the losses. The stock closed less than a percent down.

"The Amazon Washington Post has gone crazy against me ever since they lost the Internet Tax Case in the U.S. Supreme Court two months ago. Next up is the U.S. Post Office which they use, at a fraction of real cost, as their “delivery boy” for a BIG percentage of their packages...." Trump tweeted Monday.

He added: "....In my opinion the Washington Post is nothing more than an expensive (the paper loses a fortune) lobbyist for Amazon. Is it used as protection against antitrust claims which many feel should be brought?"

The Supreme Court ruled in June that states have the right to collect potentially billions of dollars in taxes from internet sales — a decision that left Amazon largely unaffected.

Trump has long called for greater taxes on Amazon, saying in January that the company's dominance is hurting American retailers. Amazon already collects sales tax in every state on the products it sells directly, which account for roughly half of all units sold on its site.

Trump has also repeatedly dinged the company for what he says is an unfair pricing structure with the Postal Service. In April, Trump signed an executive order establishing a task force to look at the operations and finances of the USPS and recommend reforms.