Automate daily payment reminders and approvals with Google Sheets and Gmail

Description

Quick overview
This workflow checks a Google Sheets invoice register every weekday at 9:00 AM, emails Gmail payment reminders for overdue unpaid invoices, and appends each reminder to a Google Sheets log for auditing.

How it works
Runs every weekday at 9:00 AM based on the configured cron schedule.
Reads all invoice rows from the Invoices sheet in Google Sheets.
Filters for invoices that are past due, have a status of Unpaid/Pending/Overdue, include required fields, and have not already been marked as reminded.
Sends a plain-text payment reminder email through Gmail to the customer email address for each eligible invoice.
Appends the reminder details (including days overdue and reminder date) to the Reminder Log sheet in Google Sheets.

Setup
Add Google Sheets OAuth credentials and replace YOUR_INVOICE_SPREADSHEET_ID with your spreadsheet ID in both Google Sheets steps.
Add Gmail OAuth credentials for the mailbox you want to send reminders from.
Create and populate the Invoices and Reminder Log sheets with matching headers (for example: InvoiceID, CustomerName, CustomerEmail, AmountDue, Currency, DueDate, Status, and ReminderSent).
Confirm your n8n timezone and adjust the cron expression if needed to ensure the check runs at 9:00 AM local time.
Decide how you will prevent repeat reminders (for example, updating ReminderSent to Yes/Sent after review, since the workflow does not update the original invoice row).