This client may be having a temporary setback and would appreciate the opportunity to arrange a payment schedule.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
Unfortunately, we still haven't received your payment of <$amount> on invoice number <invoice number> dated <date of original invoice>. Because your account is <period of time> past due, we <consequences of having an overdue account>.
If you are unable to pay the full amount due today, please call <name> at <phone number of contact> to arrange a payment schedule. If you have already sent us your payment, please disregard this notice. We value your patronage and appreciate your prompt attention to this matter.
encl: <List of enclosed items goes here>