The holder of a credit sale contract, including any financial services loan company, may, if the contract so provides, collect a late charge on each installment in default for a period not less than ten days in an amount not in excess of $50 or five per cent of each delinquent installment, or portion thereof which remains unpaid on the due date agreed upon in the contract whichever is less, and the late charge shall not be assessed more than once for the same delinquent installment. The seller or other holder shall give the buyer written notice of the assessment of the late charges prior to the due date of the next contractual payment. No late charges shall be assessed after the acceleration of the maturity of the contract. In addition to the late charge, the contract may provide for the payment of costs of the action together with reasonable attorney’s fees where the contract is referred for collection to an attorney or licensed collector who is not a salaried employee or officer of the holder of the contract.