The governing board of a municipality shall be authorized to allow and pay the regular periodic billings of any municipal utility or of any public utility whose rates are fixed by, and are subject to the approval of, the Oklahoma Corporation Commission or any other public regulatory body or board of any utility cooperative for services furnished by the utility to the municipality without the issuance of a purchase order and filing of an invoice by such municipal or public utility, provided, further, that no municipal or public utility shall attach a late charge or deny a discount on any bill until a thirty-day period has lapsed from the receipt of the bill.