I hear the "the USPS is forced to pre-fund their retirement" argument all the time. The fact is that EVERY business in the US since Enron has been forced to pre-fund their retirement. The result is that employers now have more honest retirement benefits because they HAVE TO. It's why we now have primarily third party 401K.
You can't just promise what you can't pay in the future. Why, it's pretty darn dishonest to take the retirement funds you promise your employees and re-invest them in your own (failing) business model. Actually, if you look at the numbers, the USPS is now paying more in retirement than salary even though they haven't even fully funded their prerequisite retirement fund. So basically USPS is paying for today's retirees and for tomorrow retiree's because 30 years ago they DIDN'T pay for today's retirees, they just took the money and ran the post office to the ground. If the USPS went bust today, you'd have about 20 years worth of people that won't get their retirement funds at all because the funds aren't there.
The USPS is failing because they are still controlled by government, they have to provide services that aren't profitable and their unions suck them dry through Cadillac insurance and pension benefits. Every President changes the leadership, that's not news, during Obama, it was full of Obama cronies that used the USPS to enrich themselves and report on Republican and religious organizations, who allowed Amazon to get a sweetheart deal from the USPS so Amazon has effectively been subsidized by the USPS? Obama.
If they were able to have an independent leadership, based on merits, not on politics and tune their pension and other employment benefits to match their courier services, they'd be a lot better off.