How to Automate Invoice Reminders with AI and Google Sheets

The Problem: Chasing Payments Is a Time Sink

If you've ever run a small business or done freelance work, you know the drill. You send an invoice, the due date passes, and then you're stuck writing follow-up emails one by one. It's tedious, easy to forget, and honestly a bit awkward.

I had a spreadsheet full of invoices — some paid, some overdue — and I was spending time every week manually checking dates and sending reminder emails. It wasn't complicated work, but it was the kind of repetitive task that just eats into your day.

So I decided to automate it.

The Solution: AI-Generated Google Sheets Script

The idea is straightforward: take a Google Sheet with your invoice data, use AI to generate a script that reads it, finds the overdue ones, and sends personalised reminder emails automatically. The whole thing runs inside Google Sheets — no extra apps, no subscriptions, no code to host.

I used Claude AI (Anthropic's free tier) to write the script, but you could just as easily use ChatGPT or Google Gemini. The key is giving the AI the right context about your spreadsheet structure so it generates something that actually works first time.

Step-by-Step Walkthrough

What the Script Actually Does

The final script is more capable than you might expect from something AI-generated in a few minutes:

• Scans your spreadsheet for invoices past their due date or marked as "Overdue"
• Skips paid invoices automatically
• Generates personalised emails with the client's name, invoice number, amount, and days overdue
• Shows an editable preview so you can tweak the email template before sending
• Sends all emails in one click through your Gmail account
• Logs the send date back to your spreadsheet so you have a record
• Includes a debug tool if something doesn't look right

Download the Files

Want to try it yourself? Grab both files below. The spreadsheet is a template you can fill in with your own invoice data, and the script is ready to paste into Google Apps Script.

Tips If You're Trying This Yourself

• Give the AI a screenshot. It sounds simple, but showing Claude (or ChatGPT) exactly what your spreadsheet looks like eliminates most "column mismatch" errors.
• Test with fake data first. Before pointing this at real client emails, populate a few rows with your own email address and test the flow end-to-end.
• Gmail has daily sending limits. Free Gmail accounts can send about 100 emails per day via Apps Script. If you're on Google Workspace, that limit is higher.
• The template is fully editable. You can change the email subject and body right in the preview dialog before sending — no need to touch the code.

If you want to go beyond reminders, the guide on AI prompts for UK small business bookkeeping is a good follow-up for categorisation and month-end workflow prompts.

The Bigger Picture

This is one small automation, but it's a good example of what's possible when you combine a basic spreadsheet with AI-generated code. The entire workflow took about 20 minutes to set up, and it saves me time every single week.

Late payments are a recurring issue for smaller firms, and business coverage from BBC Business often reflects how cash-flow pressure affects day-to-day operations.

If you're doing any kind of repetitive financial admin — categorising expenses, generating reports, sending payment confirmations — chances are there's a similar automation waiting to be built. And you don't need to be a developer to do it.

This is the first tutorial in a series I'm building at WealthFlow Agents. If you found it useful, subscribe on YouTube to catch the next one.