n8n AI Invoice Approval Workflow: GPT-4o Mini + Gmail + Telegram

Auto-approve vendor invoices with AI—then notify everyone in Gmail and Telegram

This n8n AI Invoice Approval Workflow receives invoices via a webhook, uses OpenAI (GPT-4o Mini) with your policy data from Google Sheets to decide whether an invoice can be auto-approved or needs human review, logs every decision, and sends outcome notifications through Gmail and Telegram.

What this workflow does

• Receives invoice submissions via a POST webhook request.
• Normalizes and validates required fields—specifically checks that vendor email, invoice number, and amount are present.
• If required fields are missing, it sends:
  • a Telegram alert (intake error), and
  • an email notice via Gmail to alert finance.
• If valid, the workflow uses OpenAI (GPT-4o Mini) and consults Google Sheets tables (VendorMaster and ApprovalPolicy) to produce:
  • an approval decision (auto-approve vs human review),
  • suggested approver,
  • reason, and
  • confidence.
• Applies deterministic rules to force human review when the amount is over 500,000 or when the vendor is not in the known-vendor list.
• Calculates due dates and approval deadlines, then appends the invoice record and decision details to the Invoices tab in Google Sheets.
• Sends notifications via Gmail and Telegram:
  • confirmation for auto-approved invoices to vendor and finance, or
  • review request to the suggested approver and alerts to finance for manual review.

Use cases

• Finance teams that want faster invoice processing by automatically approving low-risk invoices.
• Operations workflows that need policy-driven approvals using vendor and approval rules stored in Google Sheets.
• SaaS operators who want auditable logging of every invoice, decision, and reason.

Technical details

• Webhook for invoice intake
• if, set, and code nodes for validation, parsing, and rules
• OpenAI (GPT-4o Mini) to generate approval decision, reason, and confidence
• Google Sheets integration for VendorMaster, ApprovalPolicy, and Invoices logging
• Gmail for email notifications
• Telegram for instant alerts and intake-error messages