Automate Purchase Approvals in n8n: Sheets, Gmail & Slack

Automate Purchase Approvals in n8n—Policy checks, multi-level approvals, and an auditable trail in Google Sheets

This n8n workflow automatically routes purchase requests for approval based on vendor, budget, duplicate risk, and amount thresholds—then generates a purchase order and updates Google Sheets and Slack from start to finish.

What this workflow does

• Receives and validates requests from an n8n Form, checking required fields, ensuring amounts are positive, and validating an optional “needed by” date.
• Enriches with policy context by reading vendor and budget data plus recent requests from Google Sheets.
• Flags policy conditions such as unpreferred/new vendors, over-budget requests, and potential duplicates from the last 30 days.
• Calculates totals and approval routing, deciding whether the request is auto-approved or routed through L1 to L3 approval levels based on amount thresholds and any policy flags.
• Creates an approval request with auditability: for routed requests, it generates a request ID, appends a submitted entry to a Google Sheets audit log, and sends the first approver decision request via Gmail.
• Processes approver decisions through a webhook, rebuilds request state from the Google Sheets audit log, and ignores duplicate/late decisions.
• Escalates or resolves: approved requests continue to the next level (L1→L2→L3) by email; rejected requests trigger an email outcome to the requester.
• Finalizes with purchase documentation: upon final approval (or auto-approval), it creates a purchase order in Google Docs, writes the PO text, emails the PO link, logs the approval to Google Sheets, and posts an approval message to Slack.

Use cases

• Streamline procurement approvals across teams using L1/L2/L3 thresholds and policy checks.
• Reduce duplicate or risky spend by flagging potential duplicates from the last 30 days in Google Sheets.
• Keep an always-current audit trail for every purchase request, approval, and purchase order.

Technical details

• Integrations / nodes: if, set, code, Gmail, Slack, webhook
• Data layer: Google Sheets for vendor/budget data and a complete audit log
• Document generation: Google Docs purchase order creation and PO text insertion