A retail installment contract, which otherwise conforms to the requirements of this chapter, may contain the provision that the seller may at his option add subsequent purchases made by the buyer to the contract, and that the total price of the goods or services covered by the contract shall be increased by the price of such additional goods or services, and that all finance charges and installment payments may at the seller’s option be increased proportionately, and that all terms and conditions of the contract shall apply equally to such additional goods or services. The contract may also provide that the goods purchased under the previous contract or contracts

shall be security for the goods purchased under the subsequent contract but only until such time as the total of payments under the previous contract or contracts is fully paid.