The USPS is about $100 billion short in funds for retiree healthcare. Operationally they're slated to run out of cash shortly and that's going to get worse as they have to directly fund more and more retiree health care.
...specifically because they were forced to pre-fund pensions to an absurd degree by a Republican Congress. And the sole purpose of this was to damage the USPS so that it looked like it was doing much worse than it really is, so that they could push for privatizing it.
They barely made any of those payments and the rule was rescinded several years ago. The $100 billion shortfall is because they didn't make those payments and the ongoing cash flow issues are continued degredation of the USPS economic reality