The holder of a retail installment contract, upon agreement with the buyer, may extend the scheduled due date or defer the scheduled payment of all or part of any installment or installments. In any such case, the holder may restate the amount of the installments and the time schedule therefor, and collect as a refinance charge for the extension or deferment, a flat service fee not to exceed $15 and a total additional charge on the balance being extended not exceeding an amount equal to one-twelfth of the annual percentage rate originally charged on the agreement for each month the payments on the agreement are being extended or deferred. [1957 c.625 § 25; 1981 c.552 § 2; 1995 c.519 § 6]