The holder of a retail installment sales contract may, upon agreement with the buyer:

(1) Extend the scheduled due date or defer the scheduled payment of any installment payment under the retail installment sales contract; and

(2) Charge and collect a reasonable flat service fee for such extension or deferment in addition to the time-price differential calculated for the period of such extension or deferment at the rate originally agreed upon in the retail installment sales contract on the outstanding balance.