Automate Overdue Invoice Follow-Ups: OpenAI + Gmail + Sheets

Automate overdue invoice follow-ups using OpenAI, Gmail, and Google Sheets

This n8n workflow checks your Invoices Google Sheet every morning, reviews each overdue client thread in Gmail, and uses OpenAI to decide whether to send the next stage reminder—defer it—or route it for human review. The chosen action is then written back to Google Sheets so nothing falls through the cracks.

What this workflow does

• Runs daily at 9:00 AM (America/Chicago) and reads the Invoices tab in Google Sheets.
• Selects only eligible invoices (not paid/void/closed, not on manual hold, not already in human review, and overdue long enough for the next reminder stage).
• For each eligible invoice, searches Gmail for recent correspondence with the client and builds a compact thread history (including dedupe checks for an exact stage subject and detection of recent activity within the configured window).
• If the exact stage email subject already exists, it updates the sheet as already reminded.
• If there was recent conversation, it defers the next follow-up and records the decision.
• Otherwise, it sends the thread history to OpenAI to classify the context as SEND, DEFER, or HUMAN, then safely parses/validates the JSON with a confidence threshold.
• If SEND, it generates and sends a deterministic stage-based reminder email via Gmail and writes the sent stage + next follow-up date back to Google Sheets.
• If DEFER or HUMAN, it updates Google Sheets accordingly for later handling.

Use cases

• SaaS operations teams that need reliable overdue invoice chasing without manual checking.
• Automation engineers building an AI-assisted email follow-up pipeline with audit-friendly state in Sheets.
• Businesses that want to avoid duplicate reminders and intelligently pause when clients respond.

Technical details

• Integrations/nodes: Gmail, Google Sheets, OpenAI
• n8n components: if, code, no op, sticky note (used to control logic, parsing/validation, and workflow readability)
• Core features: stage-based deduping, recent-activity deferral, JSON parsing with confidence threshold, and decision logging back to Google Sheets