Telling the cashier they undercharged you is a small moment of courage nobody else will see. You’ll know.

Nobody is watching, which is precisely what gives the moment its weight. Integrity is mostly built out of instances where no version of the story would ever be told, and how you handle those quietly sets what you expect of yourself. The amount is almost never the point.

Say something when the change is wrong in your favour. It takes one sentence and a small amount of awkwardness. What you are actually buying is the ability to trust your own account of who you are.