Once they've threatened your life, all the niceties are out the window (heh). Shut the window. Call your manager. If she keeps causing shit, threaten to call the police

If you happen to land the sub-human garbage who thinks it is appropriate to get angry in public over nothing, and they get to the point where you can at least say you were afraid for your life, then you don't have to play ball with them anymore. Most people are going to side with you anyway (though I suppose that would be the shitty thing about a drive through. At least in the places where I have worked, I could play my cards right and turn everybody against the person who was angry. Maybe they will start honking if she gets loud enough.)