The bottom line is that congress forced the USPS to pay for 75 years of benefits in advance.

A Manufactured ‘Crisis’: Congress Can Let The Post Office Save Itself Without Mass Layoffs Or Service Reductions

Congress passed the Postal Accountability and Enhancement Act of 2006 (PAEA). Under PAEA, USPS was forced to “prefund its future health care benefit payments to retirees for the next 75 years in an astonishing ten-year time span” — meaning that it had to put aside billions of dollars to pay for the health benefits of employees it hasn’t even hired yet, something “that no other government or private corporation is required to do.”

As consumer advocate Ralph Nader noted, if PAEA was never enacted, USPS would actually be facing a $1.5 billion surplus today:

By June 2011, the USPS saw a total net deficit of $19.5 billion, $12.7 billion of which was borrowed money from Treasury (leaving just $2.3 billion left until the USPS hits its statutory borrowing limit of $15 billion). This $19.5 billion deficit almost exactly matches the $20.95 billion the USPS made in prepayments to the fund for future retiree health care benefits by June 2011. If the prepayments required under PAEA were never enacted into law, the USPS would not have a net deficiency of nearly $20 billion, but instead be in the black by at least $1.5 billion.