AI Automation
Overdue Invoice Chaser is an independent finance automation project designed to reduce the repetitive work involved in following up on unpaid invoices.
Built with n8n, QuickBooks, Gmail, Airtable, and Slack, the workflow runs on a daily schedule to identify overdue invoices and coordinate follow-up activities.
It uses rule-based scheduling to send payment reminders at 1, 3, and 7 days overdue, with human escalation at day 14. Airtable provides an audit trail of workflow activity, while a separate Slack alert workflow helps surface execution failures.
The implementation includes retry handling and failure notifications. I tested the alerting behavior by deliberately introducing invalid credentials and documenting how the workflow responded.
The project also documents known limitations involving missed scheduled runs and potential duplicate reminders during manual reruns.
Overdue Invoice Chaser is a self-initiated technical case study, not a commissioned or production-deployed client automation. It demonstrates integration and orchestration of finance-related business systems using deterministic workflow rules rather than an LLM.
Project details
Context
Following up on overdue invoices is a repetitive but important part of managing business finances.
When customers do not pay invoices by their due dates, businesses need to identify outstanding payments, determine when reminders should be sent, track previous follow-up activity, and decide when an unpaid invoice requires human intervention.
Managing this process manually can involve repeatedly checking accounting software, preparing reminder emails, updating tracking records, and notifying the appropriate person when an invoice remains unpaid.
Overdue Invoice Chaser explores how these activities can be coordinated through a scheduled automation workflow.
I designed and built the project using n8n to connect QuickBooks, Gmail, Airtable, and Slack.
QuickBooks provides invoice information, Gmail handles follow-up emails, Airtable records workflow activity, and Slack is used to notify operators when execution failures occur.
The workflow uses defined overdue intervals to determine when reminders and escalation activities should occur.
An important part of the project was exploring what happens when an automation does not execute as expected. I incorporated retry handling, created a dedicated failure-alert workflow, and tested alert behavior with deliberately broken credentials.
I also documented known limitations involving missed-day handling and duplicate reminders during manual reruns.
The project is an independent workflow demonstration rather than an automation deployed for a paying client.
Overdue Invoice Chaser was developed to demonstrate how multiple business tools can be connected to automate a clearly defined operational process.
The workflow follows predetermined scheduling and escalation rules. It does not depend on an AI model to determine which invoices require reminders or when those reminders should be sent.
The project also examines workflow reliability.
Sending reminders successfully is only one part of the problem. An automation must also account for execution failures, external service errors, and the possibility of repeated or missed actions.
I incorporated failure notifications and documented limitations rather than presenting the workflow as a fully production-hardened finance system.
Businesses need to identify unpaid invoices after their due dates.
Manually checking invoice statuses requires repetitive effort.
Payment reminders need to follow a consistent schedule.
Staff need to know when an overdue invoice should move from automated reminders to human follow-up.
Follow-up activity should be recorded so previous actions can be reviewed.
Failures involving external services or credentials need to be visible to the workflow operator.
Retry behavior needs to be considered when dependent services are temporarily unavailable.
Missed scheduled runs can affect the timing of follow-up activities.
Manually rerunning a workflow can create duplicate actions if execution history is not handled correctly.
Contribution
I independently designed and implemented Overdue Invoice Chaser using n8n, QuickBooks, Gmail, Airtable, and Slack. My work covered workflow orchestration, invoice follow-up scheduling, email automation, activity logging, escalation rules, retry handling, failure notifications, and documentation of known operational limitations.
The project focused on connecting the stages of overdue invoice follow-up into a repeatable automation.
Rather than having a person manually review invoices and decide which reminder to send each day, the workflow evaluates invoice information against predefined overdue intervals.
The implementation also includes supporting functionality for activity tracking and operational error visibility.
I approached the project as a workflow engineering exercise, considering both the successful execution path and situations where integrations or scheduled executions could fail.
Designed an n8n workflow connecting accounting data, email communication, activity tracking, and operational notifications.
Organized the automation around a daily execution schedule and defined invoice follow-up intervals.
Integrated QuickBooks as the source of invoice information used to determine which outstanding invoices require follow-up.
Connected invoice data to the workflow's scheduling and reminder logic.
Implemented rule-based follow-ups at 1, 3, and 7 days overdue.
Used Gmail to coordinate reminder emails according to the defined schedule, reducing the need for repeated manual preparation of follow-up messages.
Included an escalation stage for invoices that remain overdue after 14 days.
The workflow distinguishes routine automated follow-ups from situations requiring direct human attention.
Integrated Airtable to maintain a record of invoice follow-up activity.
The logging component provides an audit trail that can support review of workflow actions and previous follow-up attempts.
Implemented retry handling for workflow failures and created a separate Slack notification workflow for execution errors.
This provides a mechanism for drawing attention to failures instead of allowing them to remain unnoticed.
Tested the error-notification behavior by deliberately introducing invalid credentials.
Used the resulting failures to verify that the alerting workflow responded and to examine how the automation behaved when an external integration could not complete successfully.
Documented known issues involving missed scheduled days and potential duplicate reminders during manual reruns.
Identified these as areas requiring additional safeguards before relying on the workflow for production financial operations.
Overdue Invoice Chaser was developed independently, without a commissioned client or production deployment.
The project demonstrates practical automation engineering across multiple business services, combining scheduled execution, rules-based decision-making, external integrations, activity logging, and failure notifications.
The implementation also illustrates the difference between a functioning demonstration and a fully hardened production system, particularly around duplicate prevention and recovery from missed executions.
Implementation
Overdue Invoice Chaser uses n8n as its orchestration layer, connecting QuickBooks for invoice information, Gmail for payment reminders, Airtable for activity logging, and Slack for execution failure notifications.
The primary workflow runs daily and uses predefined overdue intervals to determine which follow-up action is appropriate.
A separate failure-alert workflow provides visibility into execution problems. The implementation also incorporates retry handling and documents limitations that would need to be addressed before production deployment.
Configured an n8n workflow to run on a daily schedule.
Each execution processes invoice information and evaluates whether outstanding invoices have reached a defined follow-up interval.
This approach separates the automation's execution schedule from the business rules determining when specific reminders or escalations are required.
Daily scheduled execution.
Invoice status evaluation.
Overdue interval checks.
Conditional follow-up actions.
Coordination of accounting and communication services.
Integrated QuickBooks to retrieve invoice information for the follow-up process.
The workflow uses invoice status and overdue timing to determine which records qualify for scheduled actions.
QuickBooks remains the source of accounting information, while n8n coordinates the follow-up activities associated with unpaid invoices.
QuickBooks integration.
Invoice information retrieval.
Overdue invoice identification.
Evaluation of follow-up intervals.
Connection between accounting records and automation rules.
Implemented scheduled payment reminders through Gmail.
The workflow uses predefined intervals at 1, 3, and 7 days overdue to determine when reminder actions should occur.
These rules provide a consistent follow-up sequence without requiring someone to manually review and initiate every reminder.
The scheduling is deterministic rather than AI-generated, making the intended reminder behavior dependent on explicit workflow conditions.
Gmail integration.
Day 1 overdue reminder.
Day 3 overdue reminder.
Day 7 overdue reminder.
Rule-based reminder selection.
Automated follow-up coordination.
Included an escalation stage for invoices that remain overdue after 14 days.
This stage distinguishes between routine automated payment reminders and cases that require direct human attention.
The escalation rule forms part of the overall follow-up schedule, ensuring that the automation does not treat every overdue invoice as a situation that should be handled indefinitely through reminder emails alone.
Day 14 escalation rule.
Identification of unresolved overdue invoices.
Separation of automated reminders and human intervention.
Conditional escalation logic.
Integrated Airtable to maintain an audit record of invoice follow-up activity.
The logging stage provides a way to review actions performed by the automation rather than relying exclusively on individual workflow execution records.
Recording activity is particularly relevant for finance workflows, where previous reminders and escalation events may need to be checked when investigating an invoice's follow-up history.
Airtable integration.
Follow-up activity records.
Audit logging.
Visibility into workflow actions.
Support for reviewing previous automation activity.
Implemented retry handling for workflow operations that encounter failures.
Developed a separate Slack alert workflow to notify operators about execution errors.
Separating failure notifications from the main invoice-processing workflow provides a dedicated mechanism for surfacing unsuccessful operations.
The implementation emphasizes making integration failures visible rather than assuming that scheduled workflow execution always completes successfully.
Retry handling.
Workflow failure detection.
Dedicated failure-alert workflow.
Slack notifications.
Visibility into external integration errors.
Tested the failure-alert functionality by deliberately introducing invalid credentials.
These tests caused integration operations to fail, allowing me to examine whether the dedicated notification workflow responded as expected.
This provided practical verification of the alerting path under deliberately induced failure conditions.
The testing did not establish comprehensive production reliability or guarantee recovery from every possible integration failure.
Deliberate credential failures.
Testing of integration error behavior.
Verification of Slack failure alerts.
Examination of unsuccessful workflow executions.
Documentation of observed behavior.
Documented limitations involving scheduled executions that are missed and workflows that are rerun manually.
A missed daily execution may prevent an invoice from receiving a reminder at the intended interval.
Similarly, manually rerunning an execution may cause a reminder to be sent again if the workflow does not reliably recognize that the action has already occurred.
These are important considerations for finance automation because repeated payment reminders can create confusion, while missed reminders can undermine the intended follow-up schedule.
I documented these limitations as unresolved aspects of the demonstration rather than claiming that the workflow provides complete idempotency or automatic recovery from missed runs.
Identification of missed-schedule limitations.
Documentation of potential duplicate reminders.
Consideration of manual rerun behavior.
Recognition of incomplete duplicate prevention.
Documented reliability improvements needed before production use.
Outcomes
Overdue Invoice Chaser demonstrates an automated approach to managing overdue invoice follow-ups across multiple business applications.
The project delivers a scheduled n8n workflow connecting QuickBooks, Gmail, Airtable, and Slack, with reminder stages at 1, 3, and 7 days overdue and human escalation at day 14.
It also incorporates activity logging, retry handling, and a dedicated failure-alert workflow that I tested using deliberately invalid credentials.
Because the project was self-initiated and not deployed for a paying client, its outcomes are engineering deliverables rather than measured business improvements.
The known limitations around missed scheduled executions and duplicate reminders remain documented. No claims are made about production readiness, reduced payment delays, recovered revenue, or actual business usage.
Deliverable
Built an n8n workflow that connects QuickBooks invoice information with automated Gmail follow-ups using predefined overdue intervals.
The workflow demonstrates how invoice monitoring and follow-up communication can be coordinated without manually initiating each scheduled action.
Deliverable
Implemented a structured reminder sequence at 1, 3, and 7 days overdue, followed by a human escalation stage at day 14.
Measured impact
Three scheduled reminder intervals and one human escalation interval.
The reminder and escalation stages follow explicit business rules rather than AI-generated decisions.
Deliverable
Integrated QuickBooks, Gmail, Airtable, and Slack through n8n to coordinate accounting information, follow-up emails, audit logging, and operational error notifications.
Measured impact
QuickBooks, Gmail, Airtable, and Slack, orchestrated through n8n.
Each service has a defined responsibility within the automation, avoiding the need for a separate custom application to demonstrate the connected workflow.
Deliverable
Implemented Airtable logging to record invoice follow-up activity and provide a reviewable history of workflow actions.
The logging component supports investigation of previous automation activity but is not presented as a guarantee against duplicate reminders.
Deliverable
Created a separate Slack alert workflow to notify operators when automation executions encounter errors.
The notification workflow provides operational visibility beyond the primary invoice-processing sequence.
Deliverable
Tested the failure notification path using deliberately invalid credentials to trigger integration errors and examine the workflow's response.
The tests verified alert behavior under induced failure conditions. They do not establish comprehensive reliability across every possible failure scenario.
Deliverable
Identified and documented limitations involving missed scheduled runs and potential duplicate reminders when workflows are executed manually more than once.
These limitations remain important considerations before using the workflow in production. Complete duplicate prevention and missed-run recovery are not claimed.
Deliverable
Developed a self-initiated automation demonstrating scheduled execution, finance-system integration, conditional business rules, activity logging, escalation, and error handling.
The project was not commissioned or deployed by a paying client. No revenue recovery, operational cost reduction, or real-world customer outcome metrics are attributed to it.