There is a demand for locally printed whistles for communities where ICE is or forecast to be.
If you are interested in printing whistles for your area, search “ICE whistles [your area]”. You can use either your state or major nearby city. If you are interested in printing whistles, but can’t find anything, let me know and I can see if I can find a local org you can support.
Regarding putting anything in mailboxes - it is actually a federal crime to put something in a mailbox unless it is properly addressed for delivery through the postal system, or you are a postal worker. I would refrain from doing that.