That 3/4" rule for delivery confirmation has been mentioned on the board a lot in the past. (Not important to you, but if it went out Priority Mail, I don't think it has to be 3/4" or higher. I think you are lucky that they didn't just put postage due on it for priority mail.)

I can only guess the reason for the 3/4" or more rule -- maybe it is to relieve the postal delivery man from the task of carefully scrutinizing all his mail for delivery confirmation (that he has to scan in with his belt computer) -- he only has to look closely at the thick (3/4") mailpieces. It also prevents scanable deliv confirmation mail from sticking together and being missed if they are too thin.

Robert