Don't consider it a relapse. Did you go out and intend to drink it your recovery be damned? Myself, I was going to buy a real beer and opted for the NA instead. It has kept me off real beer. Now I'm beginning work on changing habits not just quitting not alcohol. NA is a part of that for me at the moment. Had I bought a 6 pack of real beer it would have been a relapse. A 6 pack of NA was a safer alternative for me and I feel a large part of keeping me from real alcohol. Just don't do it again but don't take away credit from yourself by calling it a relapse!