We have a client that owes us money. The invoice originally sent in May 2022. There have been edits made to the invoice, for the client to match up with PO's. The company still has not paid us in full for the project. What is the legal interest rate we can charge for the past due balance? When can I back charge the interest to? Arizona Prompt Pay Obligation says 18% annually, but not sure if the client falls under the Prompt Pay Obligation.