Once a robbery has started, your responsibility is to survive safely. The best way to do this is to comply with the demands of the robber. If you resist, you will greatly increase your risk of being killed or injured. Be prepared to give a robber your best customer service. This is one cash transaction you'll want to conduct without any bother.
Robbers bring weapons to ensure they have control of the situation.
If you don't see a weapon, but the robber says or indicates that he has one, believe him. Let the robber think he has control. However, if you are properly trained and prepared, it is you and not the robber who will control the situation. Talk to the robber and tell him what he can expect from you. Don't leave him guessing, don't make unexpected moves.