One rude customer at your summer job will teach you more about keeping your cool than a dozen nice ones.

Composure cannot be practised in easy conditions. A pleasant shift teaches you the job; somebody being unreasonable at the counter teaches you what you do when you are embarrassed in public and cannot answer back. That is a skill with uses everywhere, and it only gets built in exactly those moments.

After the next bad interaction, give yourself two minutes and a sentence about what you did well. Then decide one thing to do differently next time, like keeping your voice lower. Treat them as the hard reps rather than as ruined afternoons.