The holder of a home improvement installment contract, upon agreement in writing with the buyer, may extend the scheduled due date or defer the scheduled payment of all or of any part of any installment or installments payable thereunder. The holder may charge and contract for the payment of an extension or deferral charge by the buyer and collect and receive the same, but such charge may not exceed an amount equal to one per cent per month simple interest on the amount of the installment or installments, or part thereof, extended or deferred for the period of extension or deferral. Such period shall not exceed the period from the date when such extended or deferred installment, or part thereof would have been payable in the absence of such extension or deferral, to the date when such installment or installments, or part thereof, are made payable under the agreement of extension or deferment; except that a minimum charge of one dollar ($1) for the period of extension or deferral may be made in any case where the extension or deferral charge, when computed at such rate, amounts to less than one dollar ($1). Such agreement may also provide for the payment by the buyer of the additional cost to the holder of the contract of premiums for continuing in force, until the end of such period of extension or deferral, any insurance coverages provided for in the contract.