Congress should call the USPS's bluff.
Binding arbitration created that situation.
If USPS can't get high enough rates to pay the cost of handling any particular class of mail Congress will have to pass legislation that allows USPS to IGNORE the Postal Rate Commission (which has of late denied any rate increases or changes).
If USPS can't come up with the money to pay 75 years in advance for future retiree health insurance premiums, Congress, which imposed that remarkable rule, will have to pass legislation that brings USPS back into line with the rest of the government and with the private sector.
Congress could also help out by passing beneficial legislation to require USPS to pay retirement deposits at the same rate as the other government agencies (rather than at a higher rate).
Congress is the problem. Congress will have to act.