US President Donald Trump on Saturday attacked the online retail giant Amazon for a second time in a week, accusing the company of scamming US Postal Service (USPS) over cheaper shipping costs.

“While we are on the subject, it is reported that the US Post Office will lose $1.50 on average for each package it delivers for Amazon. That amounts to Billions of Dollars,” Trump tweeted on Saturday.

“If the P.O. ‘increased its parcel rates, Amazon’s shipping costs would rise by $2.6 Billion.’ This Post Office scam must stop,” he wrote on Twitter, urging the retail giant to pay real costs and taxes.

It would cost Amazon $1.46 more to ship an average package through USPS if costs were “fairly allocated,” according to a Citigroup analysis released last year.

The latest attack comes two day after Trump claimed that Amazon’s shipping practices have negatively impacted retailers and local governments.

“I have stated my concerns with Amazon long before the Election. Unlike others, they pay little or no taxes to state & local governments, use our Postal System as their Delivery Boy (causing tremendous loss to the U.S.), and are putting many thousands of retailers out of business!” Trump tweeted on Thursday.

Trump’s wealthy friends have also complained to the president that “Amazon is destroying their businesses,” news website Axios reported last week, adding the president was obsessed with Amazon and considering ways to rein in the company’s power.

“He’s wondered aloud if there may be any way to go after Amazon with antitrust or competition law,” a source who has spoken to the president told Axios.

But the White House said Wednesday that the administration currently proposes no specific policies to regulate the online retail giant.

“We have no announcements and no specific policies or actions that we’re currently pushing forward or considering taking,” White House press secretary Sarah Sanders said.

According to The Hill, Amazon does collect such taxes on products sold to consumers in states with sales taxes in place, but the company doesn’t collect taxes in all states on behalf of third party sellers that use Amazon as a marketplace.