If they deliver the letter to the return address, what prevents you from writing the target address as the return address and getting it delivered for free?
Germany's new stamps aren't stamped any more - a QR code gets scanned and logged instead - so people thought they were lucky when they received unstamped letters and decided to re-use the stamp. They got hit with a fine.
[1]: https://amp.cnn.com/cnn/2024/01/13/business/uk-post-office-f...