If an invoice and two previous notices have been ignored, with no effort to make partial payment, it's time to make a stronger statement. Point out that you regret having to take this step.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
You have failed to pay your outstanding balance of <$amount>. I'm sure you do not want to damage your credit record, but since you have failed to respond to our last two overdue notices, or to arrange any sort of payment schedule, we are forced to <consequences of nonpayment>.
We regret having to take this drastic action. To avoid it, please contact our office immediately to arrange a payment schedule.
encl: <List of enclosed items goes here>