I mail a fair amount of physical media using the postzegelcode. If you put a return address, PostNL just returns the letter with a sticker on it saying the postage was not enough. They won't deliver it.
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...