Automate Supplier Invoice Processing with Document AI, Accounting API & Slack

Automate Supplier Invoice Processing end-to-end with Document AI, Accounting API validation, and Slack alerts

This n8n workflow receives supplier invoices via webhook, extracts invoice data using a document extraction endpoint, validates the results against supplier and open purchase orders from an accounting REST API, and then auto-posts approved bills, queues exceptions for review, or sends rejection alerts via Slack.

What this workflow does

• Ingest invoices via webhook: Accepts an invoice payload from your backend-connected source (e.g., email/Drive/upload) through an n8n webhook trigger.
• Extract invoice fields: Calls a REST endpoint (e.g., /documents/extract) to extract supplier, TRN, invoice number, totals, confidence, and line items.
• Validate against accounting data: Fetches supplier master data and open purchase orders from an accounting REST API (e.g., /accounting/suppliers and /accounting/purchase_orders), then checks rules including:
  • Duplicate invoice number
  • TRN format and TRN match
  • Line totals vs subtotal
  • Total vs subtotal plus VAT
  • Confidence threshold
  • PO match and approval limits
• Route the outcome: Automatically approve, send to review, or reject based on validation results.
• Act and respond: Posts approved bills to /accounting/bills, creates review items in /review/queue, or triggers rejection alerts via an alerts endpoint (Slack-backed). Returns a 200 JSON response with the outcome.

Use cases

• Reduce manual AP work by auto-processing invoices that match an existing PO and pass validation.
• Catch data issues early (VAT/total mismatches, duplicate invoice numbers, low confidence extraction) and route them to a review queue.
• Improve supplier compliance by validating TRN format and match before posting bills to your accounting system.

Technical details

• HTTP endpoints: Document extraction (/documents/extract), accounting REST API (suppliers, purchase orders, bills), review queue (/review/queue), and alerts.
• Slack: Used to send rejection alerts for invalid invoices.
• n8n nodes: webhook, set, code, merge, and switch for routing logic (plus supporting sticky note documentation).
• Setup requirement: Provide a reachable HTTPS base URL for your backend APIs and update it in the Config and Config (errors) steps.