If the customer does not answer any of your requests, you can make a third request or a final demand. You should review the history of the account, give a deadline for payment, and let customer know what you are going to do. However, you must not threaten legal action if you do not intend to take it because it will make you look weaken and indecisive. If the third request fails to bring results, never send another letter asking for payment. You must follow through with your stated action.