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?

The bill is issued on the 3rd.
It is due 8 days later, this means that it is due on the 11th.
There is a one day grace period.

Since Frank paid the bill on the 12th, this means that his payment is received after the deadline. However, he will not be charged for any extra fees as he was still in the grace period when he paid.