If a cashier made a error in your favor would you correct it or run?

I would go back and at least try to make the situation right. It isn't worth the compromise of your inegrity! :~)

the profit margins and levels of wastage on which many stores operate, may have some impact on the morality question.


This point has nothing at all to do with the ethics or morality of the question.

Clearly, the only right thing to do is return the funds in some way.

The issue I am raising is the difference between "morality" as a matter of personal integrity, and "morality" as a control device of social systems.
(Micro vs macro morality). Such issues may be "handleable" at the level of a $12 error, (right and wrong are not in dispute) but not at the level of "the duty of killing" in a military conflict.

I am suggesting that "personal integrity" is selective and parochial, and this is the wider significance of the OP.