A home improvement installment contract may provide for the payment by the buyer of a delinquency and collection charge on each installment in default for a period of not less than ten days in an amount not in excess of five per cent of such installment or five dollars ($5), whichever is less: Provided, That only one such delinquency and collection charge may be collected on any such installment regardless of the period during which it remains in default. A contract may also provide for the payment of court costs actually incurred and of attorney's fees not exceeding twenty per cent of the amount due and payable under such contract if it is referred to an attorney not a salaried employe of the contractor or holder for collection.