Recently a friend sent me a postcard of what is claimed to be the smallest Post Office in the USA which is located at Ochopee in Florida (in the "everglades", 100km south of Naples). Although little more than the size of the average garden shed, it has a thriving philatelic business providing a service to locals and stamp collectors around the world.
Ochopee Post Office, Florida 33943 U.S.A.
I wrote and sent a stamped envelope to the postmaster at Opochee, asking for an example of the postmark. It was returned in little over a week, duly postmarked and with a souvenir cachet. Amazing what a little imagination and committment can do!