Donald Trump. File pic

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.