The post office was forced into this because their unfunded pension fund was a time bomb waiting to happen. They are only paying this increase till 2016 and have had it reduced when it was pressing. As of 2009 it was estimated their unfunded liabilities were over fifty billion dollars.
No, where Congress gets a failing grade is similar to how base closings are done. Just like the military knows which bases are not needed the Post Office can tell you which sorting centers, distribution hubs, and which Post Offices, are not needed. When they go to close them then suddenly every Congressman becomes an expert and you end up with stories about how the PO wanted to close nearly 3000 offices and only got a little over a hundred.
The PO operates under burdensome contracts combined with quickly shrinking sources of income. The number of pieces of mail handled has steadily declined but when the PO tries to downsize Congress interferes or their contracts block them. Trying to hire part time workers is another area they have difficulty with.
So, no their problems don't stem from just having to pay for liabilities they should be paying for; if anything ask Congress why that rule ain't applied to the US as a whole; its from a myriad of items of which two largest are Congress and the unions.