The ICOAMP was one of those fluxus words that didn’t catch on. It was the time of the ‘Mad Cow desease’, where lots of cows were killed because they had the desease. This organisation was there to protect the cows against the mad people.
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