Cal. Bus. & Prof. Code § 17760

The provisions of this chapter do not apply to nonprofit merchant organizations that guarantee redemption of trading stamps that are exclusively issued and redeemed by that organization or its members and where all of those members have jointly and severally assumed liability for all stamps so issued and where the redemption is from stock in trade of, or in cash from, any and all members of the organization. The assumption of liability shall be evidenced by a written agreement between the organization and its members setting forth the joint and several liability of the organization and its members for redemption of trading stamps issued by the organization.