You are wrong.
The fact it's based on my experience - shows it is real advice. Not what I think but what I expereienced.
Take the timber frame. I measured it to get a quote. The company then measured it "properly". When it was delivered was wrong!. But they measured it wrong not me. Imagine if that had been the other there way around - liability would of been with me. I'm probably going to have 40k worth of glass. If a company ( after my initial measurements ) can't be bothered to send a sales rep to measure ( so they accept liability ) ; then I'm not going to risk 40k.
You could argue measuring for windows is a much simpler thing i.e. the size of the hole. Quite probably if you measured yourself all would be good.