Its a problem of government but not in the traditional way. Its monopolies that are inefficient (public or private) and most agencies are monopolies. But USPS is not, they have several strong competitors.
2005, USPS generated over 1B in profits. So they were doing well (or at least ok). But because its a public agency, USPS is susceptible to the whims of politicians. Who in 2006, decided it was a good idea to saddle them with an artificial expense, exceeding 5B per year for 10 years. Now this money isn't actually spent, so USPS borrows it from the treasury and then pays it back, to the treasury.
But it still counts as a 'loss' so they still have to make changes to try to cover the loss. If they were a private company, they wouldn't have this weird bill and even if they did, they could raise prices and cut services. But because they're a public agency, they have to ask permission to raise prices*. And they are susceptible to pressure not to close post offices. So with no visible cuts allowed, they have to make invisible cuts. Like close and reduce hours at sorting centers, creating Rec's problem.
Now the good news is, the 10th year is almost here so new payments to nowhere will end. The bad news is, the 50B+ they cumulatively borrowed from and given back to the Treasury, is still on the books. But they can at least start reporting operational profits again. And if the 2006 requirement is erased, the debt it created gets erased too.
* on US->US delivery. they've been raising prices on US->intl for a while, making US companies less competitive.