(Keep in mind that I have some bias here, because I am speaking of the USPS while I work for the USPS.)
There is a sign in the lunch room of my postal warehouse that has a quote from the UPS: "The Postal Service wouldn't last one day in the free and open market of competition."
Now on the face of it, that is true. The amount of people at my workplace who fail to put in a solid effort would not be possible in a privately-run business, because private business, for the most part, involves the profit motive, which means squeezing every possible dollar out of every possible thing, cutting away all ease for the sake of productivity and efficiency. At a place like UPS or FedEx or Amazon, one must go and go and go and go, every minute a maximum effort.
UPS is implying that without being subsidized by the government, the USPS would not be able to compete with this output. But the USPS is not a private company buoyed by government subsidies; it is a federal agency. It does not run on the profit motive. It does not need to operate in the black. It needs to get its job done. Efficiency and self-sufficiency are nice because they mean the budget-cutters in Congress aren't breathing down our necks, but the USPS has been able to keep going through the deacdes that it was legally required to shove a crapton of its budget into funding pensions 70 years in advance, which would kill a private company.
And the one job that the USPS absolutely needs to get done is vital to all the citizens of the United States. Because the only way to get direct individual communication from the government is through the mail. Not by phone or email, unless you've specially arranged that with one agency or another; not by text at all. The stuff you usually communicate with the government involves personally identifying information, and the only way to send that securely is in an envelope.
Now, imagine putting the USPS into the free market while keeping this responsibility in their hands. It would be a disaster. Private business is subject to all sorts of nonsense because of the profit motive. Mergers, acquisitions, layoffs to bump up the stock prices, corner-cutting, cost-cutting, mismanagement, shareholder returns valued above all else, and all of it coming at the cost of product quality. Some of you may have personally experienced the way FedEx delivers packages these days, by chucking them in the vague direction of your door and speeding off. I wouldn't be surprised if that was because drivers were facing increasing pressure to deliver as fast as Amazon -- and those poor Amazon drivers are forced to deliver fast indeed. Imagine if the USPS acted like that! Letters scattered all over the lawn, count on it.
Whatever twerp at the UPS thinks the USPS should be in the free market is missing the point. UPS and USPS are in different lanes. And, honestly, I'd rather be in the USPS lane, even if it means people slacking off at work. The fact that they CAN slack off at work means I'm in a better place than the poor saps at UPS.
EDIT:
Also, I'm sorry about helping you get your junk mail, but getting paid by companies to help them advertise is one of the ways the USPS earns money. Better that than total reliance on the federal budget.