It's not an official federal agency funded by tax dollars, it's an independent agency funded solely by its own revenue stream (aka stamps and other services, at least up until 2006).
It's the only service that guarantees delivery to every American regardless of location. UPS, FedEx and other express carriers, including companies with independent upstart logistics wings like Amazon, ALL use the USPS for last mile delivery when they don't want to do it themselves because it's too much work.
It's only running a massive deficit because of the "accountability" act that was passed in 2006 by the GWB administration that requires them to prepay pensions for 50 YEARS, as well as forbids them from building revenue in any way unrelated to delivering mail (meaning they can't innovate or raise money in any way other than more efficient delivery of the mail, while running billions of dollars of deficit due to the pension issue). Before that, it was generating a profit.
The only reason the USPS is "failing" is because some members of the federal government want it to fail, because they'd make more money by outsourcing its services to their donors.
Aaaaannddd..... doing the GOP's second favorite thing. Fucking over poor people, along with their third favorite thing, killing a giant union.
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"If you wish to converse with me define your terms" Voltaire