If its pressed against your head. Make a grab for the gun. If theres a counter dive and draw firing through or over the counter. The act of fighting back is sometimes enough. If it ain't. Keep fighting.
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.
Lol. I could also immagine a couple of old air bags strategically placed under floor tiles or wall of the counter.