[mikeash]

That works, until the plane really is full and you need a new one, or your letter carrier really is fully occupied and you need a second person.

It makes little sense to give nearly everyone free shipping, but charge every 100,000th person $50,000 because their item was the one that needed a new plane. So instead, they divide the cost across all the stuff that gets shipped so everyone pays a share. That per-item cost is typically more than what these sellers are charging, thus the questioning.

[gohrt]

There is no single optimal way to divide the cost among all the passengers. See also: airlines, and capitalist vs socialist economies.