You can stamp "non-machineable" on the envelope but there is no guarantee that it won't be machined. I have had several differnt chips mailed to me in one of those corrugated inserts in a regular envelope stamped "non-machineable" and it was obvious that it have been processed like a regular envelope. I've had good luck using a #000 bubble envelope with a dogface butterfly non-machinable surcharge rate stamp on it which costs 0.70 cents.