>> I cannot find any documentation to indicate that the USPS does not have large unfunded liabilities, <<
It looks like a $2.8 billion unfunded liability here: http://www.usps.com/financials/_pdf/annual_report_2009.pdf
Wonder what FedEx and UPS have on their books?
You have been fooled by dishonest government accounting. Congress and the USPS are at odds over unfunded pension and retiree health care liability with each side claiming that the liability belongs to the other side. This dispute is just between two parts of the government. It is essentially a blame game to avoid responsibility. The USPS is part of the federal government regardless of any rhetoric or language that Congress uses to separate the USPS and federal government.
Here is the bottom line for taxpayers. There is a $100 billion unfunded liability. I believe that $100 billion is a large understatement of the true liability given the number of postal retirees. Neither the USPS nor Treasury Department has a portfolio to support pension and health care benefits for postal retirees. If the USPS were a private company (not part of government), it would be required to have an investment portfolio to fund its retirement benefits.
Congress spends every dime it collects plus it borrows heavily. The USPS is no different than any other government agency except that it collects lots of revenue through postage fees. Congress has not created any mechanism to fund pensions for postal employees or any other government employees.