If you react with a fight response, that means your instincts recognise you're up against a foe you can defeat. A flight response means your instincts think you can escape in time. Freezing is what your body does when it can't defeat or escape a threat.
In a confined space like a dairy, chances are you're not going to be able to flee.
That leaves fight or freeze.
If you have a weapon nearby and you're able to defend yourself, you might react with a fight response. But that would depend on where you are in the store and where your weapon is located.
If it's behind the counter and you're stacking shelves, you're not going to reach it in time.
Chances are, you'll freeze.
All the training in the world can't prevent what happens when your instincts take over.
This is why I think it's more sensible to focus on prevention rather than arming shopkeepers - what are the chances that the weapons will even be useful?
I'd rather avoid a robbery than rely on my untested instincts.