There is something you CANNOT do and is very, very illegal. Most people would assume that this is illegal, but if you commit this crime, you are committing a felony in the United States.
With all of the junk mail we get these days can you imagine having to pay for it? Back in the days before stamps were invented it wasn’t the sender, but the receiver of a letter who paid for it. But back then people really sent letters and that’s usually the only thing you would get in the mail. Bills and junk mail came much later.