While how it's run is often dictated by congress, the USPS does not use government funds. It is self-sufficient (correct term?).
You think you're going to be able to send a simple document through UPS or FedEx for 46 cents? Also, it's an overall pain in the ass. If you privatize, they will just have to hire more people to handle the volume, pay for more gas to go into more vehicles, pay for more health insurance, etc. You think their rates will go down or stay the same? I know there are FedEx and UPS workers who post on this site. I'm not putting them down or placing letter carriers above them. I'm just saying that privatizing is not the way to go.