When the US Postal Service first began, it couldn’t even be called that, since there wasn’t a US yet. There was, of course, a postal service, but it was the Crown Post and it was subject to interception and investigation, and we really couldn’t have that and a revolution, now could we?
In fact, there were competing services to the Crown Post, but there were also gaps in that service and general inefficiencies. For instance, most services transported parcels at a lower rate than letters. So, people worked around the rule by attaching letters to small, mostly empty parcels just to save money. One service, run by a man named Benjamin Mumford, carried both Royal Mail and that of his own service, which was cheaper than the King’s rates.
About the artwork: that’s a picture of a 2006 stamp issued commemorating Ben Franklin’s tenure as the first Postmaster General. It’s marked “B. FREE FRANKLIN” because that’s how he often signed his name while in that position. Why? Because as the Postmaster General, he had franking privileges and didn’t have to pay his own postage. It was a privilege he took great advantage of.