California Code, Labor Code - LAB § 3731

Any stop order or penalty assessment order may be personally served upon the employer either by (1) manual delivery of the order to the employer personally or by (2) leaving signed copies of the order during usual office hours with the person who is apparently in charge of the office and by thereafter mailing copies of the order by first class mail, postage prepaid to the employer at the place where signed copies of the order were left.