Gmail Supplier Invoice OCR with OpenAI + Google Sheets Checks

Automatically extract supplier invoice data from Gmail PDFs and validate it against your Google Sheets ledger

This n8n workflow monitors a Gmail label for supplier invoice PDFs, uses OpenAI to extract structured invoice fields, then runs validation checks in Google Sheets—posting approved invoices to the ledger, routing large invoices to Slack for sign-off, and returning failures to the supplier.

What this workflow does

• Gmail-triggered invoice intake: Watches a specified Gmail label for new emails with attachments.
• PDF filtering + text extraction: Keeps only PDF attachments and extracts the PDF text.
• OpenAI-powered OCR to structured fields: Converts invoice text into structured data such as supplier, invoice number, PO number, dates, currency, totals, and line items.
• Google Sheets checks (8 validations): Loads supplier records, open purchase orders, and already-posted invoices from Google Sheets, then checks for issues including duplicates, totals/tax validity, PO matching, and invoice date sanity.
• Ledger posting or escalation:
  • If all checks pass and the invoice amount is within the approval limit, it appends the record to the Google Sheets Ledger.
  • If checks pass but the amount exceeds the approval limit, it posts a Slack message requesting human sign-off.
• Failure handling: If any check fails, it appends the invoice to the Google Sheets Exceptions sheet and emails the supplier with a numbered list of issues.

Use cases

• Automating supplier invoice OCR and validation for AP teams using Gmail + Google Sheets.
• Preventing duplicate invoice postings by comparing invoice data against already-posted ledger entries.
• Ensuring purchase order (PO) alignment and invoice date reasonableness before recording expenses.

Technical details

• n8n nodes/integrations: Gmail, OpenAI (extraction), Google Sheets (ledger, open POs, suppliers, exceptions), Slack (approval escalation).
• Logic controls: Uses nodes such as if, code, filter, and sticky note for conditional routing and processing.
• Setup: Configure Gmail label ID (with attachments enabled), add OpenAI credentials, and provide the Google Sheets spreadsheet URL and sheet names for the ledger and exception handling.