More to this point, they are required *by law* to pre-fund retiree medical benefits ahead of retirement for all their employees. It costs several billions per year! no other organization in America, public or private, has that burden https://twitter.com/MattBruenig/status/1249697670509314050 …
The thing that gets me is that a lot of private businesses prefund their medical liabilities, but when they do that they usually invest the funding in corporate bonds to get some return... Post office is banned from that, they can only invest in treasuries that yield nothing