The customer yelling about ketchup is not your enemy. They’re your practice for harder people.

Most of the anger that arrives at you in a service job started somewhere else and found a convenient target. Knowing that does not make it pleasant, though it does change what it is. Handling one badly behaved stranger calmly is practice for much harder versions later, in rooms where more is at stake. The skill transfers even when the job does not.

Keep your voice level and fix whatever can be fixed. Afterward, say out loud to somebody that it was unpleasant, because it was. Then let it belong to the day rather than to you.