I'm here at Great Clips, in a packed waiting area, and this dude walks up to the counter, while facing all of us, realizes his fly is down, and he zips in front of us.
In this situation, my gut feeling is that you own it, pay your bill, and readjust in the parking lot. He went with full frontal zip. Kind of confused if he made the right call.
Also relevant to point out this occurred in Missouri. That may change the response.