n4 Topa proffers no plausible argument that a charge of six months' interest on the principal represents "the result of a reasonable endeavor by the parties to estimate a fair average compensation for any loss that may be sustained" for late payment of an interest installment. (Garrett, supra, 9 Cal. 3d at p. 739.) The contract, moreover, expressly provides for a reasonable late payment fee equal to 10 percent of the payment.