This is a difficult letter to send, but sometimes necessary, especially for small businesses or companies offering services. You risk losing the customer in the future, but a few more like him could put you out of business.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
This is to acknowledge that we received your payment of <$amount> on invoice number <invoice number>. Unfortunately, it was <amount of time payment was late> late. Our policy states that payment must be made <policy>.
In a business our size we're constantly forced to monitor cash flow. In order to maintain the quality of <type of products or services you provide> we provide, we must pay our suppliers, often before we can serve you.
In the future, we must require that you pay for all <products/services> in advance. We will be happy to review our position after <time period>. Thank you for your understanding.
encl: <List of enclosed items goes here>