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USPS ships about 40 percent of Amazon’s packages. Amazon bulk-delivers packages to a USPS distribution center, and the Postal Service brings it to your door. USPS negotiates the discounted rate for that service with Amazon, as it does all other bulk shippers.
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In 2017, parcels brought in $19.5 billion, or 28 percent of USPS’ annual revenue. At $2.1 billion, packages contributed the largest revenue increase.
Deals with private shippers like Amazon accounted for $7 billion of the $19.5 billion in revenue. While we know that Amazon is the biggest e-commerce player, we don’t know exactly how much of the $7 billion comes from Amazon, because the details of the postal service’s deals with private shippers are considered proprietary and not made public.
I'd assume Amazon is a fairly large chunk of that 7 $Billion
https://www.politifact.com/truth-o-m...osing-fortune/