Any person who enters into a retail installment contract, charge agreement, or lender credit card agreement that does not comply with the provisions of this chapter or who violates any provision of this chapter except as a result of an accidental or bona fide error shall be barred from the recovery of any service charge, official fees, or any delinquency or collection charge under or in connection with the related retail installment contract or purchases under a retail charge agreement or lender credit card agreement; but such person may nevertheless recover from the buyer an amount equal to the cash price of the goods or services and the cost to such person of any insurance included in the transaction: PROVIDED, That if the service charge is in excess of that allowed by RCW 63.14.130, except as the result of an accidental or bona fide error, the buyer shall be entitled to an amount equal to the total of (1) twice the amount of the service charge paid, and (2) the amount of the service charge contracted for and not paid, plus (3) costs and reasonable attorneys' fees. The reduction in the cash price by the application of the above sentence shall be applied to diminish pro rata each future installment of principal amount payable under the terms of the contract or agreement.