In the 80-ies I was experimenting with selfmade artistamps. Tried them on envelopes and let them run through the system. Seems the system worked quite well. In the beginning years of mail-art I even numbered outgoing mail-art. This was enveloppe number 1000. After a few years I quit with that numbering
This enveloppe was sent to J.F. Chapelle in France. beside the stickers on the enveloppe I also included a handmade envelope that I made with my 3D Pen. Plastic can be used to make your own one-of-a-kind objects.