(a) Whenever a marketer discovers or has called to its attention a billing error or other mistake acknowledged or admitted to by the marketer and resulting in an overpayment by a retail customer of said marketer, such marketer shall be required automatically and immediately to provide:

(1) A credit of the amount of the overpayment to the account of the customer; or

(2) A refund of the amount of the overpayment to the customer.

(b) A marketer shall be prohibited from requiring a retail customer to whom it owes a credit or refund to submit in writing a request for such credit or refund before the marketer complies with the provisions of subsection (a) of this Code section. All credits to the account of the customer or refunds to the customer shall be made within 60 days after the overpayment has been acknowledged or admitted to by the marketer.