I think the post office could have been this, but the political will wasn't there. Separation of banking from taxation and the postal system from both of those for separation of concerns for compartmentalization was probably at the forefront of the minds of the founders, since Washington himself had run a spy network and been personally hunted by soldiers and mercenaries on their own turf during the revolution, so I can't say these aren't legitimate concerns, but they haven't exactly aged well. At the time of the revolution and directly after the union, there was no federal income tax anyway.