“I would think,in Amazon’s case. because of its volume of use”
That works for manufacturers, economy of scale. The USPS I highly doubt, the more they ship the more they lose in all probability.
Having said that, I probably receive more packages with Amazon Contract “White Panel Van” Delivery Service than the USPS now.
Currently they give free home delivery, and charge to pick it up there. Ya don't see any pizza joints doin that do ya?