Legal Question in Business Law in California

What you can charge depends on your agreement with them. More to the point, why are you continuing to do business with them under the circumstances you describe?

IMPORTANT NOTICE: The above response is not intended to, and does not, create an attorney-client, fiduciary or other confidential relationship with the responder. Neither does it constitute the providing of legal advice or services or the giving of a legal opinion by the responder. Such a relationship can only be created, and legal advice and/or legal services provided, pursuant to a written agreement with the responder. Accordingly, no obligations of any kind are assumed with respect to any matter or question presented. It should also be noted that legal issues are often time sensitive and legal rights may be lost or compromised if you do not act in a timely fashion.

If your contract does not provide for late charges, interest, etc., you probably lack any legal basis to bill for and collect such charges and fees, but nothing prevents you from trying. If your contract does, however, provide for late charges and/or interest, you are legally entitled to enforce those provisions if they were a reasonable estimate of your damages at the time the contract was made. Unreasonable charges or usurious interest cannot be enforced.

Mr. Whipple is right. Charging late fees is usually a matter of contract. It depends on what is in your contract.