A gas company or electric company may bill its customers for gas, electricity, or any other service it renders only on the basis of the net total cost of the service under the applicable rate that is filed for that service. The Commission may authorize a gas company or electric company to apply an additional charge over the net total cost to any bill or part of a bill that is not paid: within 20 days for a residential customer or residential cooperative; or within 15 days for any other customer. The additional charge that is applied by a gas company or electric company under this subsection may not exceed 5% of the net bill or part of the bill. Unless the Commission approves the imposition of different charges on different classes of customers, any additional charges applied by a gas company or electric company under this subsection shall be uniform for all customers.