No person shall charge, collect or receive from any buyer, directly or indirectly, any further or other amount for costs, credit investigation charges, insurance premiums, examination, appraisal, service, brokerage, commission, interest, discount, expense, fee, fine, penalty or other thing of value in connection with a home improvement installment contract other than the charges authorized by this act. Any such unauthorized charge shall be unenforceable. Any payment thereof shall be applied to the next maturing installment, or, if the contract has been fully paid, remitted to the buyer and the buyer shall be entitled to recover all such unauthorized charges.