n8n Invoice Approval Automation: GPT-4o + Google Sheets

Automatically approve invoices from PDF submissions—powered by GPT-4o, Google Sheets, and Gmail

This n8n workflow receives invoice PDFs via a webhook, extracts and validates invoice data using OpenAI (GPT-4o), checks for duplicates in Google Sheets, and routes each invoice for automatic or manual approval—while notifying vendors and finance via Gmail.

What this workflow does

• Receives invoices via POST webhook including vendor details and the PDF file.
• Extracts structured invoice fields by pulling text from the PDF and using OpenAI to convert it into amounts, dates, line items, and payment information.
• Normalizes and validates required fields; if key fields are missing, it emails the vendor through Gmail.
• Detects duplicates in Google Sheets by searching for an existing invoice with the same invoice number. Duplicates are flagged and finance is notified by email.
• Analyzes categorization, priority, and approval need using OpenAI, including a check that the total matches subtotal + tax.
• Auto-approves or requests approval: low-value, low-risk invoices are auto-approved; others trigger an approval request and wait for a decision submitted via webhook.
• Writes results back to Google Sheets with the final approval status and sends a completion notice to the finance team via Gmail.

Use cases

• Finance teams that want invoice approval automation without manually copying PDF data.
• SaaS operators needing duplicate invoice prevention using Google Sheets as the source of truth.
• Operations workflows where vendors must be contacted automatically when invoice fields are incomplete.
• Teams that require a human-in-the-loop approval flow based on GPT-4o risk/priority assessment.

Technical details

• Integrations/credentials: OpenAI (two AI steps: extraction + analysis), Gmail (vendor, approver, finance, admin notifications), and Google Sheets (duplicate checks + final record writing).
• Key workflow nodes: if, set, code, wait, gmail, switch.