If I had a convenience store, I would rig the counter with some sort of anti-kneecap device that could be triggered during a holdup. Something to knock 'em down so you can casually walk around the counter and do what needs to be done.
There was a convenience store in I believe the Philippines that had barred windows and door and the owner was being robbed and electronically locked the door as he ducked out the back.
The cops came and collected the fool stuck inside the store :-)