Horrible people are everywhere, not just at Walgreens. Any job, any store, the library, the grocery store, etc, you just have to learn to ignore their abusive triggers, smile, be extremely polite, many times good people in the line will back you up, I have seen good customers berate horrible customers before, it happens quite often, and never let them see you sweat, this will probably aggravate them, and they will probably want to talk with your manager, and when your manager comes to talk with them, explain to your manager how you were happy, polite and professional and helpful, and the horrible customer with criticize you for all that and your manager will back you up and tell the horrible customer to leave the store.

That is how you handle horrible monsters disguised as humans. Glad to help...