California Code, Civil Code - CIV § 1810.10

(a) Notwithstanding the provision of any contract to the contrary, except as provided in subdivision (b) or (c), no retail seller shall assess any finance charge against the outstanding balance for goods purchased under a retail installment account until the goods are in the buyer's possession.

(b) A finance charge may be assessed against the outstanding balance for such undelivered goods, as follows:

(c) In the case of a home improvement contract as defined in Section 7151.2 of the Business and Professions Code, a finance charge may be assessed against the amount financed from the approximate date of commencement of the work as set forth in the home improvement contract.