New Delhi, March 18 -- Amazon is preparing to significantly reduce the number of parcels it routes through the US Postal Service (USPS), a shift that could reshape the economics of last-mile delivery in the United States and have ripple effects for businesses and consumers alike.

Amazon, historically the Postal Service's largest customer, is seeking to cut its reliance on USPS by as much as two-thirds by the autumn, when its current delivery contract expires. The company has already begun reducing the volume of packages it hands off to the agency, according to people familiar with the matter.

The scale of the shift is substantial. USPS handled more than one billion Amazon packages last year, accounting for roughly 15 per cent of its tot...