Cal. Bus. & Prof. Code § 17538.6

(a) It is unlawful for any person conducting business in this state to require or request a consumer to issue a postdated check unless (1) the person accepting the check advises the consumer in writing that the check may be cashed immediately, notwithstanding the postdating, unless the consumer files a postdating order with the consumer’s bank pursuant to Section 4401 of the Commercial Code, and (2) either of the following occurs:
1. (A) Receipt of the advice is acknowledged by the consumer in writing.
2. (B) The advice is clearly printed on an invoice for goods or services that is provided to the consumer at the same time that the check is solicited.
(b) This section shall not apply to any person who requires or requests a consumer to issue a postdated check if the recipient of the check does not submit the check for collection or cause it to be submitted for collection until on or after its date.
(c) As used in this section:
1. (1) “Bank” means any person engaged in the business of banking and includes, in addition to a commercial bank, a savings and loan association, savings bank, or credit union.
2. (2) “Check” means a draft, other than a documentary draft, payable on demand drawn on a bank, even though it is described by another term, such as “share draft” or “negotiable order of withdrawal.”