Reflecting back this weekend about my first job as a teenager (fast food, gasp, the horror!) When I was doing this and remember being on the front line, the personalities, demeanor and attitudes varied GREATLY in the clientele I would encounter.
One of the things that I use in situations when the other party is proving to be a challenge, is treat them like my mom. The idea here is to be kind, patient and understanding instead of getting stressed and/or upset.
Maybe there is another person to insert here (grandparent, dad, sibling, etc). If you come across a similar situation, do escalate the level, but rather take a deep breath and treat them how you would treat that person.