I think I have said this a million times on this forum, but here I go again: remain calm, remain calm, remain calm. Keep your cool when you are on the clock. If you work here long enough, you will experience supervisors who have anger issues. Not if, but when. Sounds like you've found one of these guys. Don't forget, UPS is no different than anything else around. You've got normal people, and you've got a sackful of anti-social kangaroos. Many sups whole game plan is too make you lose your cool. This is the one thing you have to avoid. If you lose your cool, your whole case gets lost in your temper tantrum. They do something stupid, you lose your cool, and what they did gets lost in the shuffle.
Keep your cool to avoid this. Stick to the issues. Hell, even laugh at them, just don't lose your cool. Then do what you need to do. File your grievance.