It's okay to do something, when you can exploit it? Like you can't go about laundrying money. But you can paint or draw by hand small details on a piece of card and then trade them away.
https://www.flickr.com/photos/kajsab/ <--- my flickr from 2010 onwards. I used to take pretty good panoramas?!
Also that letter I wrote yesterday. I put a meowth pokemon card in it and some cut cardboard, goggly eyes and a word magnet by accident. They have no value, and like paper anyway... so I don't think I should need to use a customs stickers for a letter. I don't have them and my printer is broken since forever. Might get too bulky anyway.