You don't HAVE to you NSC envelope; I was just saying that you should use a overnight carrier (like, fedex, ups or USPS-express), so that you can track it. Take the address to be sent from the envelope (if they sent one) provided by NSC. Othewise talk to your lawyer about the address.
This is what I was told by my attorny: "Never send a reply to rfe without the ability to track it."
hth