All the business owner needed to do was refund the money. He knows exactly what he's doing and he's wrong for it.

"I'm sorry for the price difference between what you were charged and what was listed on our website. We've refunded you the $4 that we overcharged you and hope that you'll give us another chance. We're currently working to update our website so that customers like you are not mislead.

Thank you for your business."

Done. That's it. That's all that had to take place.