Approve supplier invoices from Gmail with GPT-4o-mini, Google Sheets and Telegram

Description

Quick Overview
This workflow monitors unread Gmail messages with PDF attachments, extracts invoice details with OpenAI GPT-4o-mini, checks vendors and duplicates in Google Sheets, and routes approvals via Telegram. It records outcomes in a Bills sheet, alerts finance on exceptions, and emails a weekly payment run summary.

How it works
Triggers every minute when a new unread Gmail email with a PDF attachment is found and extracts text from the PDF.
Sends the email context and extracted PDF text to OpenAI (GPT-4o-mini) and parses the response into a fixed invoice JSON schema.
Looks up the supplier in a Google Sheets “Vendors” tab and checks the Google Sheets “Bills” tab for an existing record with the same vendor and invoice number.
Automatically approves invoices that are from a known vendor, under the configured auto-approve limit, not duplicates, and above the confidence threshold.
Flags duplicates and unknown vendors for review, and sends known-vendor invoices that exceed limits or have low confidence to Telegram for Approve/Reject with a 3-day timeout.
Appends the invoice, decision, and notes to the Google Sheets “Bills” tab, marks the Gmail email as read, and notifies finance on Telegram for any non-approved outcomes.
Every Monday at 08:00, reads approved unpaid bills in Google Sheets, selects those overdue or due within 7 days, emails an HTML payment run list via Gmail, and updates the Bills rows with a PaymentRun ID.

Setup
Add credentials for Gmail, Google Sheets, OpenAI, and Telegram in n8n.
Create a Google Sheets spreadsheet with “Vendors” and “Bills” tabs and the required columns, then replace YOUR_SPREADSHEET_ID in all Google Sheets nodes.
Populate the Vendors tab with at least VendorName, VendorEmail, and AutoApproveLimit so the workflow can match suppliers and auto-approve thresholds.
Replace YOUR_TELEGRAM_CHAT_ID in the Telegram nodes with the approver/finance chat ID(s) you want to notify.
Set the recipient address in the Gmail “Email Payment Run” step ([email protected]) and adjust the Gmail search query if you want to filter by label, alias, or mailbox.