Firstly, don't name the business as things can become complicated.
Secondly, IMO, I think he is well within his rights to seek clarification and don't see it as rude. So, if he did get it wrong wrt the thickness of canvas, would you still pay cause he got it wrong? Also, when ordering anything you have to be clear and concise otherwise the chances of getting it right are high and all the risk lies with him. Put yourself in his shoes for a moment and a customer was giving you such risk, would you accept? I see nothing wrong with his request, IMO.....Good luck with getting what your after...:-)