Being kind to an angry customer at minute 59 of your shift is faith with an apron on.

The last hour of a shift is when whatever you believe gets tested, because the reserves are gone. An angry customer at that point is not a moral opportunity, it is an irritation. Staying decent anyway is unglamorous and nobody will mention it. Conviction shows up in the state you are in when there is nothing left.

Decide before Saturday what your floor is, and make it something you can hold when tired. Not cheerful, just polite. Then let yourself be properly finished at the end rather than replaying the encounter all evening.