And that $119 /yr pays for all the shipping for all the packages?
Really?
Or is the money for shipping coming from some other source?
For example I just priced a nice trackball mouse for my computer on amazon. With prime the shipping is free, without prime its $3.99. One bestbuy.com shipping for the same mouse is $5.49.
Using the USPS shipping cost estimator the cost for a similar box from similar locations is $7.20
So the real question is where is that money coming from? Is amazon eating that $3.99. Hard to believe there is that much extra profit in a $25 mouse.
$119 / $3.99 means that about 30 shipments are covered by that Amazon Prime membership. And remember that once someone has bought Amazon Prime, they are more likely to buy from Amazon, so it helps generate more revenue and profits. (There's a term for that but I can't recall it right now.) Stop and think about that 30 shipments. That's someone ordering something not even every two weeks. Pretty nice way to get a captive consumer.
USPS also benefits in a similar way, by getting lots of business from a captive consumer who has fewer customer service problems for the USPS than an Average Joe shipping something. USPS shipping estimates are based on you shipping something that probably has higher than average problems and costs than the "mass produced" Amazon shipments.
Plus, USPS uses lower-cost delivery personnel for Amazon shipments than your Average Joe $7.20 package.