A half-inch of mar-resistant Lexan or even generic polycarbonate will stop a .45 at 50'. Put bars over the door OUTSIDE so the creeps know they can't just walk in if they break the clear panel. Replacing the door with tempered glass is asking for it to be broken again. 
If people think they MIGHT be seen doing something they'll think twice, which is why even posting pics of eyes helps deter crime. Make sure CCTV cameras are visible and prominent, both inside and outside the store. Motion detector lights and alarms that are activated BEFORE entry is accomplished help too. It doesn't have to be a garish alarm. Even turning on radio music will let the perp's know they've been detected. An ounce of prevention....