Wash. Rev. Code § 63.14.050
A retail installment contract may be contained in more than one document, provided that one such document shall be 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 case such document, together with the sales slip, account book or other written statement relating to each purchase, shall set forth all of the information required by RCW 63.14.040 and shall constitute the retail installment contract for each purchase. On each succeeding purchase pursuant to such original document, the sales slip, account book or other written statement may at the option of the seller constitute the memorandum required by RCW 63.14.110.
RCW 63.14.050