There is not as many trap doors as there should be. If I owned such a business I would build a drop door at the checkout point, it would be triggered by a sequence on the register. One more possible trap door in front of the counter would keep the trash captive until someone arrived to take out the trash. Gunfighting is a last resort thing. Escape, reposition, gain some advantage or keep moving to safety, or die. If a gun is drawn on you have no advantage until you do something to gain it. Drop and roll might work if you know how to draw a gun and roll and have worked on this skill, and you have the physical/mental condition required to pull it off.
If there is a counter between you and the bad guy and you just dropped and roll the instant you see the gun then he would need to move forward and point down to shoot you. That's all the time you need if you pratice it. But if you try and invent a move when the time comes to need it, you will be thinking to much and fumble fingers will grown on your hands, the safety function will require your reading the manual and the snaps will not unsnap while you are laying on the holster and.....