Billing and Invoice Follow-Up

Create an n8n workflow with a Schedule trigger - once or twice daily is sufficient for most firms. When the trigger fires, add an HTTP Request node that calls your billing system's API to pull all open invoices. The API call returns invoice records including: invoice ID, client name, client ID, amount, issue date, due date, payment status, and assigned relationship owner. Filter this list to include only invoices that are unpaid and that haven't already been sent through the complete follow-up sequence. For each qualifying invoice, calculate the invoice age relative to today and the due date relative to today. This determines which stage of the follow-up sequence applies.

QuickBooks invoice query example:
GET https://quickbooks.api.intuit.com/v3/company/{{realmId}}/query?query=SELECT * FROM Invoice WHERE Balance > '0' AND TxnDate > '{{30_days_ago}}'

Xero invoice query example:
GET https://api.xero.com/api.xro/2.0/Invoices?Statuses=AUTHORISED,SENT&DateFrom={{start_date}}

For both: filter the returned invoices by PaymentStatus = "outstanding" and calculate DaysUntilDue and DaysOverdue for each invoice in an n8n Function node.

This is the most important step in the workflow. Before any follow-up message goes out, check the CRM for open service issues, active complaints, or disputes on the client account. For each invoice, use the client ID or email to look up the client record in your CRM. Check for any activity records tagged as "Service Issue," "Complaint," "Dispute," or equivalent. If any such record exists and is marked open/unresolved, route the invoice to the exception queue with a note: "Invoice #X for [Client] is at Stage Y of follow-up but has an open service issue: [issue summary]. Human review required before sending." This check must run on every workflow execution - not just at setup time. A service issue that wasn't logged yesterday could be logged today. The check runs each time, every time.

For invoices that pass the relationship context check, determine the appropriate follow-up stage based on the invoice's position in your timeline. Pass the invoice details and the stage to an AI node to generate the follow-up message. The AI uses your example messages for that stage as tone guides. Configure the node to output the message text and the subject line. Send the message from the relationship owner's email address (not a generic billing address) for all stages up to the partner escalation stage. Log every outbound message as a CRM activity on the client record immediately after sending. This creates a complete audit trail - if a partner needs to step in at day 21, they can see exactly what the automated system already sent before making any manual contact.

You are writing a billing follow-up message for a professional services firm. You need to match the tone and approach for the specific stage in the follow-up sequence.

Invoice details:
- Client: {{client_name}}
- Invoice amount: {{invoice_amount}}
- Invoice number: {{invoice_number}}
- Invoice date: {{invoice_date}}
- Due date: {{due_date}}
- Days overdue: {{days_overdue}} (negative means still before due date)
- Relationship owner: {{owner_name}}

Stage: {{stage_name}}
Stage description: {{stage_description}}

Example message for this stage (use for tone reference only):
{{stage_example_message}}

Write a follow-up message that:
- Matches the tone described for this stage exactly
- References the specific invoice number and amount
- Makes it easy for the client to take action (includes payment link placeholder: [PAYMENT_LINK])
- Sounds like {{owner_name}} wrote it personally
- Is appropriately brief for the stage (confirmation = 2-3 sentences, reminder = 3-4 sentences, direct notice = 4-5 sentences)

Return ONLY the email body text and a suggested subject line as JSON:
{
  "subject": "Subject line",
  "body": "Email body"
}

The most critical operational requirement is immediate sequence termination when payment is received. Set up a secondary workflow (or a path in the same workflow) that fires when your billing system reports an invoice as paid. The payment detection can work two ways: (1) real-time via webhook if your billing system supports payment webhooks, or (2) on each scheduled run, cross-reference the open invoice list against a "paid in the last 24 hours" query and remove newly paid invoices from the active sequence. When payment is detected, the workflow should immediately: mark the invoice as resolved in the tracking system, remove it from the active follow-up queue, and send a payment receipt confirmation to the client. Log the payment as a CRM activity. Test this payment detection logic thoroughly before going live. The worst outcome of billing automation is a client paying an invoice and then receiving a follow-up reminder. Run a complete payment simulation before launch.