In a revolving charge agreement, a seller may contract for and, if so contracted for, the seller or holder of the agreement may charge, receive, and collect the service charge authorized by this section. The service charge may not exceed the amount agreed to by the parties. In the event any payment by a buyer is insufficient to pay both the credit service charge and that portion of the outstanding indebtedness then due, the payments must first be applied to the credit service charge then due.