The holder of a retail installment contract, if the contract so provides, may collect a delinquency charge on each installment in default for a period of 10 days or longer. The delinquency charge for any installment shall not exceed five percent of the delinquent installment. In addition to the delinquency charge, the retail installment contract may provide for the payment of reasonable collection costs. The collection costs may include the payment of reasonable attorney fees, if the contract is referred to an attorney not a salaried employee of the holder of the contract for collection, plus the court costs and disbursements. [1957 c.625 § 7; 1981 c.552 § 1]