The post office themselves claim it's a congressional mandate to pre-pay 75-years worth of employee pensions and benefits in a 10-year window. They give Congress 5.5 billion dollars every year, and it's that which is driving the crisis.

I believe the USPS snark is that they are paying pension and benefit costs for mail carriers that haven't been born yet. How unusual, congress screwed something up. The cynic in me is yelling "conspiracy!"