The Post Office is required by legislation passed by Republicans in Congress to pre-fund health care cost for retirees over the next 75 years. The payments are $5.5 billion per year until 2016. This level of pre-funding is unknown in the public or private sectors. The Post Office also operates mostly over what it can make, relying mostly on postage, thanks to the Post Office Reorganization Act of 1971.
It is obvious that the Post Office is mainly the victim of unprecendented legislation specifically designed to bring around its demise.