Keep the first message simple
A concise factual reminder leaves room for an administrative mistake to be corrected without unnecessary escalation.
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Invoice reminder
Write a reminder that makes the next action obvious.
Identify the invoice, amount and due date, ask for payment or a clear response, provide the correct payment route and set a sensible follow-up point. Check every fact before sending.
Record delivery and response
The value is not only the wording; retain when and where it was sent and what the customer said next.