A public utility (usually just utility) is an organization that maintains the infrastructure for a public service (often also providing a service using that infrastructure). Public utilities are subject to forms of public control and regulation ranging from local community-based groups to state-wide government monopolies. (Common arguments in favor of regulation include the desire to control market power, facilitate competition, promote investment or system expansion, or stabilize markets. In general, though, regulation occurs when the government believes that the operator, left to his own devices, would behave in a way that is contrary to the community's best interests.)
If your description is correct, why should we be charged anything to use it? The USPS only receives tax dollars as subsidies for costs associated with the disabled and overseas voters. OTher than that they are on their own. The fact it that they do need to profit, that is the reason they are making cuts.