Frank's electric bill has a cycle day of the 3rd and a due date 8 days later with a one day grace period. What happens if Frank pays his electric bill on the 12th of the month?

He won't be giving any late or penalty charge. Grace period is commonly given by the company as a form of goodwill concerning the consumers who are late in disbursing their late bills. Since on the 12th day is still measured as within the one day grace period, Frank is not entitled to pay any additional amount from his normal charge of the bill (or any other kind of penalties)