Civil Code §1678.
If more than one payment made by the buyer is to constitute liquidated damages under Section 1675, the amount of any payment after the first payment is valid as liquidated damages only if (1) the total of all such payments satisfies the requirements of Section 1675 and (2) a separate liquidated damages provision satisfying the requirements of Section 1677 is separately signed or initialed by each party to the contract for each such subsequent payment.