Cal. Civ. Code § 1810
For the purposes of this article, a retail installment account agreement shall be deemed to be accepted by the buyer if (1) the buyer signs the agreement, including signing an application containing the terms of the agreement, or (2) the account is used by the buyer or a person who has been authorized by the buyer to use the account, and the buyer has been notified in writing that the use of the credit card will mean that the agreement has been accepted by the buyer. The agreement shall not become effective unless and until the buyer has been given or provided with the disclosures required pursuant to Section 1810.1, and has accepted the agreement as provided in this section. A subsequent change in any term of the agreement shall not become effective until the seller has given notice of the change and complies with the provisions of subdivision (d) of Section 1810.3.
Ca. Civ. Code § 1810