A retail installment contract which otherwise conforms to the requirements of this chapter, may contain the provision that the seller may at the seller's 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 service 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 time sale price under the previous contract or contracts is fully paid.