What is the subject of this message?

Hello, Mr. Jones. This is Marty White calling to let you know that we have cancelled your order for building supplies.  Unfortunately, you have not paid for the last three shipments. Therefore, we see no alternative other than suspending our products and services to you. Also, if you do not pay what you owe us, we will be forced to take you to court to recover the money. We regret having to take such strong action, but we see no better way to deal with this situation.