Look it up yourself.
Just Google the Postal Accountability and Enhancement Act of 2006. It's in Title VIII. It looks like the "75 year" is someones interpretation but it's also true that the USPS is required to fund retirees health care in a way that no other entity, public or private is required to do.
Social Security is a retirement plan that has never failed to make a scheduled payment and is fully solvent into the 2030's. All it would take to fix any problems it has is to raise the income cap a modest amount.