Not so fast. You're right that the general operations of the USPS are no longer directly appropriated by Congress. However, the US government and its citizens do give the USPS significant economic benefits, including a 100% tax exemption from state and local taxes, and the ability to "pay" federal taxes back to itself[1]. USPS need not comply with local zoning, parking, toll road, etc. laws.
According to a 2015 report[2] cited by Fortune, that included $2.1B in waived taxes, the benefit of a federal monopoly on deliveries worth $14.9B, and as much as $490M/yr in subsidized lower interest rates due to not having to have a credit rating and borrow on the open market, but instead getting preferential treatment from the Treasury. Not included in the report is the fact that USPS OIG and USPIS are federally funded through appropriations[3]--whereas UPS or FedEx would have to hire their own off duty police offers at significant expense to receive the same level of protection.
That's interesting, thanks for the detailed reply.My point was that their operating expenses are covered by fees for postage and not directly paid through taxes. But now I realize that this is more of a grey area than I had though.