Guys, it is pre-funding of retirement - something that no other US government agency is required to do - which is resulting in the humongous loss. It's an accounting thing, NOT an operational thing.
Whether the USPS would be profitable without having to pre-fund the retirement is debatable, but when you hear the next politician saying that they need to take a hatchet to the US Postal service, just realize that they are talking out their backsides; it was congress that required the crazy accounting in the first place.
On a side note:
Many times I have to drop off packages "after hours" at both the main post office in town and at the UPS facility. I pull up in back where the big trucks drop off and pick up mail / parcels.
Easily, the guys at the loading docks at the US Post Office in my town are working harder, cooperating more amongst each other, and more friendly to the other Joe Blows who are dropping off parcels after hours than the people at UPS.
Why is this? I don't know. Maybe they are all afraid of getting laid off. But it's been this way since I ever started dropping off packages after hours (for a few years now).