No one, because if they quadrupled their price Amazon would never use them at all.
People say that the USPS loses money on every Amazon package. Others say they make money. I think that the USPS has no idea which. So many costs for the USPS are fixed and have little or no relation to the amount shipped. Between retirement costs and current buildings, the USPS loses billions if it doesn't ship a single letter or box. They cry when they have too little mail and now they cry they have too much.
I’ve seen USPS routinely delivering Amazon packages on Sundays. There is no way USPS is making money doing that, and no way Amazon would pay USPS DT rates for that shipping.
It’s completely wrong for US taxpayers to subsidize Amazon through USPS.