
Are ya questioning the requirement that the Postal Service be required to (up-front) fund the pension fund for subsequent retirees............ 
If State and/or local governments had been required to do the same, Many states that are now into BILLIONS..............WITH A "B"........in (unfunded liabilities)..........aka California, Illinois, Wisconsin, Ohio..................etc. etc. 
All "requirements" they don't get to choose for themselves based on their own revenue have to be a question. USPS is stuck between a rock and a hard place.
Congressional/statutory requirements for types of service, days of service, number of service locations, where services have to be delivered, etc
ad nauseum. Requirements that, given revenue and requirements of their current customer base USPS wouldn't always choose for itself. But USPS doesn't get to choose what it does. Nor are rates for those services entirely of its own selection. It's easy to make fun of the post office. But those boys and girls get the shaft (of Congressional kowtowing to consituency) and then have to smile while they get it.