When I was in training for the USPS it was time to re-negotiate union agreements and I was told during that time that USPS also had contracts with Amazon to deliver their packages. That Amazon paid a flat price for USPS to deliver all packages regardless of quantity.
This is why Amazon can offer free priority shipping and have competitive prices on products. I was told that USPS loses money with the contract(makes sense) and that essentially the government subsidizes the monopoly that is Amazon.