If the customer disputes the invoice
Record the exact issue and supporting communication. Do not treat a genuine dispute as silence or allow automated wording to ignore it.
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Customer has not paid
Verify, contact, document and decide.
First check for an administrative error, missing approval, dispute, credit or partial payment. Contact the customer clearly, document the response and set a review date rather than sending disconnected messages.
If there is still no resolution
Review the evidence, amount, relationship, jurisdiction and current formal process. Obtain advice where the position is uncertain.