Mail delivery, like many internet services, has very nice economies of scale. Once you are paying the setup cost for someone to go to every door every day, the

Mail delivery, like many internet services, has very nice economies of scale. Once you are paying the setup cost for someone to go to every door every day, the marginal cost of adding one more package is not high. So USPS can probably offer any particular mail-sender very low rates and justify them with the idea that the revenue exceeds their marginal cost to deliver (though if they offer everyone such low rates they won't be covering their true overall costs).