Is it possible to send a letter without a stamp? Try this: Just swap the sender’s adress with the recipient’s address and put the envelope into the mail box without a stamp. It takes like one week, but the letter will be sent to the sender’s address for sure. Post office is not allowed to throw letters away. I once have sent socks back to a friend, who borrowed me a fresh pair. Well actually this is not legal and I don’t want you to become a trickster, but it’s interesting that still today, at times when everything seems so perfectly structured in a safe way, you still find so many holes in the system, just by behaving unrational and doing things the opposite way.