Don't pile up crates or boxes in front of windows or in aisles. You're responsibility is to survive. You will greatly increase the danger of being injured or killed if you resist. Your best defense is to comply with the demands of the robber. The money can be replaced, your life cannot.
Robbers bring a weapon to ensure they get control of the situation.
Once a robbery has started, it is too late to do anything now. Let him think he has control. Robbers are desperate people. Drugs and alcohol are often factors in robberies. Desperation, intoxication and weapons make a volatile combination.