When a customer pays a bill as submitted by a public utility and the billing is later found to be incorrect due to an error either in charging more than the published rate or in measuring the quantity or volume of service provided, the utility shall refund the overcharge with interest from the date of overpayment at the legal rate or at a rate prescribed by rule of the Commission. Refunds and interest for such overcharges may be paid by the utility without the need for a hearing and order of the Commission. Any complaint relating to an incorrect billing must be filed with the Commission no more than 2 years after the date the customer first has knowledge of the incorrect billing.