That -$3B, or whatever it's ballooned to now, comes even though a federal law makes it a felony to compete with the USPS and deliver "non-urgent" mail. That's why FedEx always claims to be delivering "urgent" messages. Here's a Baltimore Sun article from 20 years ago when armed USPS police were actively enforcing this law:
http://articles.baltimoresun.com/1994-01-07/news/1994007008_...
If you give me a government-granted monopoly and a phalanx of federal agents empowered to put my competitors out of business, I'd have to be providing very poor service to be losing billions of dollars. Unfortunately, the USPS has managed to do just that.