Take a look at Chapter Nine: A Policy of Honesty, in “The Story of the Chofetz Chaim” esp. p.100-103, where it discusses how the Chofetz Chaim made a continual kiddush Hashem by always accurately determining the postage on his sefarim before sending them to the post office.
Also, imagine if you or your child or friend or relative had a store, and something like this happened, and someone who didn’t know you had a connection to this particular store, boasted to you about how they had lucked out there because the owner/cashier made such a mistake.
Just because this cashier was not grateful at that moment doesn’t mean it didn’t occur to her later that her job could have been on the line and you saved it. Or, maybe she doesn’t like her job and couldn’t care less if the owner gets ripped off and she gets fired.
And, it’s not true that no one would ever have known: I was once undercharged–the price ticket was wrong–at a small, local store where I had never shopped before, and they traced me and called me up demanding i pay the full price of the item, because otherwise it would come out of the poor clerk’s pocket.