I just looked at my agreement and realized I do not have a late payment policy in place however, this is my wording regarding payments:

Payment for services rendered is expected in full at the agreed upon time. There will be a 7 day grace period for payment. In the event any third parties are employed to collect any outstanding monies owed by “CLIENT”, “CLIENT” agrees to pay reasonable collection costs, including attorney fees, whether or not litigation has commenced, and all costs of litigation incurred.