If I could determine from which direction the shots were coming from, I would run the other way.

If I decided to stay, I would look for cover (not just concealment) since we are talking about an active shooter. If I was not armed with a firearm, I would begin looking around for something that could be pressed into service as a weapon. Does the store you are in have a sporting goods section? Then, I would wait to ambush the
bad guy. Be alert and think aggressive. Hopefully, rescue would show up first.

I would not play dead, unless the bad guy was actually a brown bear.