They're losing money for a variety of reasons. The most important is that they are mandated to exist by congress, and are supposed to be financially autonomous, but are micromanaged by congress. You'd have to think long and hard to come up with a worse group of PHBs. Congress told them to pre-fund in full their retirement fund for the next 75 years. The USPS has basically said, "This requirement is bankrupting us. If you relax it, or let us make our own decisions we'll be fine." People wonder why they can't compete with FedEx, UPS, DSL, etc, and the answer is simply that those companies don't have to listen to Congress dictate details like telling them to pre-fund the entirety of a 20 year-old employee's pension right now. I'm all for fiscal responsibility and responsible funding of pensions, but is ten years of secure pension funding not enough? 20? 30? I mean, 75? How do you even estimate your pension needs 75 years in advance?
On another note, one idea I've heard that was intriguing would allow them to operate something like a bank. Not a financial investment house, but a low-end and low-cost branch bank. Sure, I might not switch all my finances over to it, and most people probably wouldn't either. But I might open an account and seed it with some cash if it were convenient. I could send mail and have it draw on the account without having to buy stamps or wait in line. Just drop it in a box at the post office and enter my account number/pin. It could work really nicely. There's already a branch in every city. And for a lot of working poor that have no bank affiliation, it might be the most convenient place to open an account, reducing the population of unbanked. Basically a public option for retail banking.
Believe it or not, it works like this in most other places, mostly with success. And this is the way it used to work in the US as well, but it was not FDIC insured, and was phased out in the 60s or 70s.