It has been a while since I have seen that one, but if I recall the two main points were

1 - Forced to save billions over a short period of time for retirements benefits. 2 - Inability to set their own prices, like a business would.

The extra business from Amazon should have been a massive boon to USPS. But it seems the ridiculously low rate they got into, plus the couple factors above (and I am sure many others - like being required to maintain rural routes at a loss) has put them in a bad situation.