Eh? Who mandates that and good luck getting the unions to sign off on that
pretty sure its not republicans mandating that

Republican law from 2006. There's a proposed law to repeal it that has been blocked by Republicans. It wasn't unions that added the health care funding, it was GW Bush who threatened a veto if the language wasn't in there.

The health care funding is something like $60B out of the $160B shortfall since 2007, so definitely not the whole problem... but it's a very large chunk of the problem.

The USPS is prohibited by law from charging less than cost for packages, so it's not Amazon either.

The main problem as I understand things is personnel costs, the decline in First Class mail, and requirements around where the health care money is invested.

Some possible fixes:
1) Remove the ridiculous health care funding and restrictions for something more like the rest of the government's approaches
2) Allow it to stop delivering paper mail 6 days a week. Most of it is advertising circulars and other crap. A small price hike here (currently limited to the CPI) would help.
3) Allow more competitive services
4) Forgive the present debt (something like $10B) so they can invest in capital improvements instead of just servicing debt and health care costs
5) Fix the personnel cost issue, and I suspect this is where most of the rancor will happen.