A home improvement installment contract which otherwise conforms to the requirements of this act may contain the provision that the contractor may at his option add to such contract subsequent home improvement installment sales made by such contractor to the buyer, and that the total time balance of the goods and services covered by such contract shall be increased by the principal amount financed under the subsequent sale or sales, and that all finance charges and installment payments may, at the contractor's option, be increased proportionately and that all terms and conditions of the contract shall apply equally to such sale or sales. In addition, such contract may provide for the payment by the buyer of the additional cost of premiums for continuing in force, until the due date of the final installment of the consolidated time balance, any insurance coverages provided for therein. The minimum finance charge as provided in section 301¹ may be used but once in any series of add-on home improvement installment sales.