If a retail installment transaction is a sale of goods for a cash sale price of $150 or less where no security interest is retained or taken by the seller, the retail installment contract may consist of an original document signed by the retail buyer, stated to be applicable to purchases of goods or services to be made by the retail buyer from time to time. In such a case, the original document, together with the sales slip, account book or other written statement relating to each purchase, must set forth all of the information required by Sections 3, 4, 5 and 6 of this Act and constitutes the retail installment contract for each purchase. On each succeeding purchase under that original document, the sales slip, account book or other written statement may, at the option of the seller, constitute the memorandum required by Section 21.