Although the Postal Service does not obtain direct federal appropriations (with a minor exception), its claims that it receives no government aid are deceptive because they ignore numerous, valuable implicit government subsidies. Unlike private-sector businesses, the Postal Service never owes federal income taxes, state and local income taxes, state and local sales taxes, or state and local gross receipts taxes; it is exempt from inventory and property taxes on assets it owns; and it pays a low-cost alternative to the unemployment tax. The government also subsidizes the Postal Service by granting it numerous regulatory favors and governmental powers. Some of the most important ones are exemption from motor vehicle licensing and registration requirements, immunity from parking tickets, exemption from local zoning and land use regulations, immunity from antitrust laws, and power of eminent domain. Moreover, the Postal Service's government connection enables it to borrow at a subsidized interest rate. The Postal Service's indirect government subsidies, which are sweeping even compared to those at other government owned or sponsored businesses, hide financial weaknesses at the agency and are a drain on government treasuries at the national, state, and local levels.This report estimates the magnitude of several of the Postal Service's hidden government subsidies and finds that the total dollar amount may exceed $1 billion annually. The report estimates that, on the Postal Service's operations in 1997, its income tax exemptions gave it a subsidy of approximately $500 million, about $415 million from the federal government and about $75 million from state and local governments. An estimate of the Postal Service's total income tax, sales tax, and property tax subsidies in 1997 is $1.2 billion. Moreover, because these estimates omit some tax and regulatory benefits, they understate the full amount of the government aid. If the Postal Service had lost its subsidies—that is, if it had been treated comparably to other businesses—it likely would have been forced to report a loss in 1997.
It is prohibited by law from making a profit anyway!
Regarding state and local licensure, property taxes, etc, USPS is a large enough entity that should this ever be a problem it could easily relocate all its major facilities in a state imposing such penalties to another nearby state that didn't. It is anticipated that the states would be quite competitive given the rather environmentally benign character of your typical postal operation.
That is to say, if Massachusetts wanted to tax USPS, they could move to surrounding states to avoid such a tax.
Now all of that is separate and apart from the fact USPS Is a wholly owned creation of the federal government which acted under the "post office and post roads" clause. It is simply not taxable by a state, nor can it be regulated by a state.
Regarding zoning and land use laws, USPS is subject to a federal law that encourages it to adhere to local and state laws in that regard anyway. In addition USPS long ago announced it would follow such laws to be a good neighbor. On top of that, however, federal law does allow USPS and DOD to FILL WETLANDS! So, if you want that to happen, what you do is get USPS to bring in a facility and fill those wetlands. In about 75 years you'll find that property back on the market all nice and dry!