n8n Workflow: Chase Unpaid Invoices via Google Sheets + Gmail + Slack

Chase overdue invoices automatically—without pestering clients who already replied

This n8n workflow runs every weekday morning to find unpaid, overdue invoices in Google Sheets, checks Gmail for any recent client replies to hold those cases, then drafts and sends stage-appropriate reminder emails using OpenAI and posts a single Slack digest of what happened.

What this workflow does

• Schedules and holiday awareness: Executes on a weekday-morning schedule, loads configuration values, and checks date.nager.at for public holidays to determine whether reminders should be sent today.
• Identifies eligible invoices: Reads the invoice ledger from Google Sheets and prior chase history from an n8n Data Table, then selects invoices that are overdue and eligible based on due date, status, hold_until, chase limits, and minimum gap rules.
• Prevents duplicate chasing via Gmail: Searches Gmail for recent inbox messages and holds any invoice where the client has replied within the configured window—capturing the email subject and snippet for review.
• Writes context-aware reminders: Processes invoices one at a time and uses OpenAI (GPT-5.6) to draft a nudge, firm reminder, or final notice. Tone is adjusted based on days overdue and payment history.
• Safety validation: Validates and sanitizes the draft to ensure the correct invoice number and amount are included, falling back to a safe template if needed.
• Sends and logs: When dry_run is disabled, sends the email via Gmail and upserts chase count and last-chased date into the n8n Data Table.
• Slack digest reporting: Posts one consolidated Slack summary of sent and held items (including reply snippets), plus totals.

Use cases

• SaaS finance ops: reduce manual follow-ups while automatically pausing when clients respond.
• Freelancers and agencies: send reminders at the right escalation stage without over-contacting.
• Automation engineers: maintain auditable chase history using an n8n Data Table and scheduled runs.

Technical details

• Scheduling/logic: if, set, code, no op nodes
• Integrations: Google Sheets, Gmail, Slack
• AI drafting: OpenAI (GPT-5.6)
• Holiday check: date.nager.at
• Chase tracking: n8n Data Table (upsert chase count + last-chased date)