[fml]

You should consider how much you value your time. For me, it would be about $50 per hour for such activity. Consider this: it would take 15 minutes to get to the store, about 15 minutes to find the right person at the store and explain the situation, and about 15 minutes to return home. The value of my time, by my reckoning, would be about $38. Cost of gas and wear and tear on the car would probably be another $2, so call it $40.

For starters at $50 an hour smarts why did you not notice a $20 undercharge at the time? I am willing to bet you would not only notice an overcharge, but would take the time to get your money back if you had neglected to notice at the check out.

Secondly, customer service is at the entrance - not a difficult task

[Skepolitic]

For starters at $50 an hour smarts why did you not notice a $20 undercharge at the time?

Because it's chump change. If I were to have noticed at check out, the only honorable option would be to point it out to the clerk.