It's an artificial debt... their pension plan is way oversized. Congress has put them in the ridiculous position of literally pre-paying pension obligations of employees who haven't been born yet.
USPS has authority to run its own deficits quarter to quarter or year to year, and they have significantly improved their financial situation since the financial collapse and large drop in first class mail usage. Last year they would have run a surplus if it were not for the pre-funding liabilities: http://www.nytimes.com/2013/11/16/us/postal-service-trims-it... Congress needs to get out of the way and let the USPS succeed, but I fear the privatization interests are just setting them up for massive failure.