How To Write A Good Demand Letter California . Money owed vs breach of contract. Make a copy of each letter before you send it, and keep a copy of the post office receipts (use certified mail, return receipt requested). Demand Letter from www.scribd.com The clearer you are about what the evidence is, how you know the recipient has it or controls it, and why it’s important, the better off you are if you need to explain it in court. It may be written and submitted directly by the injured party, or by the injured party’s attorney. You're a small business owner, and a customer has not paid their bill despite you sending out invoices.