Comment by JSR_FDED
There’s an even better way to send an actual letter for free.
Simply switch the destination address on the envelope with the sender address, and drop it in the mailbox.
When then post office returns the letter to sender because of insufficient postage it will have delivered the letter for you.
Keep in mind that in the US this is illegal, and it's unreliable, since insufficient postage mail isn't necessarily returned. This is one of the oldest forms of mail fraud, and they're well aware of it.