Revenue and Taxation Code § 60338

If any supplier fails to pay any weekly installment of tax shown to be due by the supplier’s weekly return on the Tuesday when required to be paid, a penalty of 5 percent shall be added thereto. In addition, if any weekly installment of tax remains unpaid on the last day of the calendar month following the month during which the last of the removals, entries, or sales occurred on which the weekly installment was levied, a penalty of 10 percent of the installment, exclusive of penalties, shall be added thereto.

The weekly installment shall be deemed not paid or unpaid on any particular day:

(a) If not paid prior to 5 p.m. of that day, when paid in person.

(b) If the envelope in which the remittance is enclosed bears a post office cancellation mark dated later than that day, when paid by mail.