The customer who yelled at you over a coupon just taught you patience at zero tuition.

Being shouted at by a stranger over something small is unpleasant and unusually instructive. You get to practice staying level while somebody else does not, in a low stakes setting where the outcome does not follow you home. That capacity transfers directly to harder rooms later: interviews, arguments, difficult calls. Most people pay a lot for that lesson and anyone working a counter gets it free.

Afterward, take a minute to notice what you did with your voice and your face. Keep whatever worked. The aim is not to stop feeling it, only to stop it running the interaction.