Since it was a computer error, I wouldn't worry too much about it. The cashier will likely get retrained, perhaps all the cashiers.
Of course, if you still feel bad about the extra money, you do have two choices. One, you can return to the store, ask for a manager, and explained what happened--LEAVING OUT that it was a mystery shop. Two, you could make a donation to your favorite charity.
"To be persuasive we must be believable; to be believable we must be credible; to be credible we must be truthful." Edward R. Murrow
Thou shalt not steal. I mean defensively. On offense, indeed thou shall steal and thou must.--Branch Rickey