What is the maximum amount of late fee that you are allowed to charge on a past due invoice for service in Maryland?

This situation sounds like it is a business to business (or business to consumer) issue. As such, there probably isn't a late fee regulation specific to your circumstances. You tend to see late fee restrictions in the landlord/tenant, mortgage lending, and consumer lending fields. In general, late fees have to be reasonable and effectively estimate the cost to the business of a customer paying late. Outside the arenas where late fees are regulated, late fees are a matter of contract law. That means, both sides, you and the customer must agree, ahead of service, that a late fee can be charged. General rules for late fees are these. 1. Before any service is rendered, the possibility of a late fee should be disclosed. Having a late fee provision on an invoice after services have been rendered opens you up to some risk that, push comes to shove, a court would deny the late fee because the buyer did not have notice that such a late fee could be charged and therefore did not agree to it (no contract for a late fee). So, what ever agreement or bid you provide to customers prior to starting services should mention a late fee and specify the terms. Even worse, it sounds like you want to add a late fee after the fact. It sounds like you don't have consent of the customer to charge a late fee, nor was the possibility of a late fee even disclosed on the subsequent invoice. What you are doing is trying to punish this customer by adding a late after the fact when the possibility of a late fee and its terms was never disclosed; few if any courts will go along that. 2. Generally, you only get one late fee. Now again, if you disclose ahead of time that the late fee will be charged for each month the customer doesn't pay, that is probably okay so long as the amount of the late fee is reasonable. However, here the answer depends on what sort of business you run. For example, web hosting is a monthly service. In that context, the service provider only gets 1 late fee per monthly invoice. They don't get to keep adding late fees for each month. However, if the business is a one off service, like a plumber, they can probably get away with a late fee that can be added for each month the customer doesn't pay. But again, the possibility of a late fee and the terms of the late fee must be disclosed ahead of providing service.