I make the cardboard so the envelope can't be folded, however I've been told that postal machines can't read the stamp "Do Not Bend" so there is further risk.

I haven't had one machine mangled yet, but the point about a "clay" chip getting cracked, does make sense. I don't know if the cardboard has protected my shipments or if I've just been lucky.

But you did give me something to think about.