Build a Custom GPT That Writes Overdue Invoice Reminders

Updated November 3, 2026. Built against the tool’s current screens and checked step by step. How we check this lesson

OpenAI is retiring custom GPTs on December 11, 2026 and replacing them with plugins. Here’s how to save yours →December 11, 2026 is the scheduled retirement date in OpenAI’s custom GPT retirement FAQ for Enterprise workspaces; OpenAI says other plans may follow the same timeline. Checked October 17, 2026. See OpenAI’s migration guide.

Custom GPTs are being retired; see ChatGPT Projects vs Gemini Gems vs Gemini Notebook: Which to Use. This lesson stays here for reference, but it is no longer in our lesson lists.

Ask plain ChatGPT for an overdue invoice reminder and you get all caps, emojis and threats of legal action. In this free lesson you build a payment reminder writer for Maple Street Bookkeeping: a Custom GPT that follows the firm's collections policy and writes a reminder that is polite, firm and matched to how late the invoice is.

Heads up: OpenAI is retiring Custom GPTs and moving them to plugins (December 11, 2026 for Enterprise workspaces; other plans may follow), and creating new GPTs is ending. This lesson still teaches the core skill (a role, clear rules and a reference document), and a ChatGPT Project is where to use it today.

Start here first: Get On-Brand Social Posts Every Time using a ChatGPT Custom GPT. This Finance version follows the same build with its own example business, Maple Street Bookkeeping.

What you will build: a payment reminder Custom GPT

You start by asking ordinary ChatGPT one bare question: write a payment reminder to a client whose $850 invoice is 45 days overdue. The reply is the email nobody should send. The subject line says URGENT and FINAL NOTICE. It calls the client a Valued Customer, says the firm is very disappointed, and threatens fees, collections and legal action within 48 hours. There is no invoice number, no due date and no way to pay.

Then you build a Custom GPT called Payment Reminder Writer. You give it written rules and a collections policy document, and send the exact same prompt again. This time it writes a real reminder: a subject line with invoice #1042, a greeting by first name, the amount, the original due date and the days overdue, three ways to pay, a request for payment within 7 days, and an offer of a short call or a payment plan. It reads like someone who wants to get paid and keep the client.

Who this is for

This lesson is for the people who chase money at a small business: the bookkeeper, the office manager, the owner who does the books on Sunday night. Maybe you work at a small bookkeeping or accounting firm and send reminders for your own invoices. Maybe you handle accounts receivable for a shop or a service business. Either way, writing the fourth reminder of the week is draining, and it is easy to sound too soft or too harsh. You do not need any AI experience. If you can copy and paste, you can do this.

What goes into the reminder rules and the collections policy

The instructions read like a brief for a new hire. Sound calm and friendly, never like a debt collector. No emojis, no all caps, no exclamation points. No threats, guilt or shame, and never the words final warning or legal action. Every email has a subject line with the invoice number, a first-name greeting, the invoice number, amount, due date and days overdue, the ways to pay and one next step, all in under 150 words.

The core of it is tone by days overdue. From 1 to 15 days it is a friendly note that assumes the client simply missed it. From 16 to 30 days it is a clear reminder asking for payment or a date. From 31 to 60 days it is polite and firm: pay within 7 days, and here is a call or a payment plan if timing is hard. At 61 days or more the GPT does not write the email at all. The owner calls the client personally.

The collections policy PDF repeats those stages and adds the payment terms (due 30 days after the invoice date; pay by link, bank transfer or check), a rule to only mention a late fee if the engagement letter includes one, a rule for clients who say they already paid, and a sample reminder. The numbers and terms in the lesson are examples. Use your own policy.

Why it works

Plain ChatGPT does not know your terms, your clients or how you like to handle late payers, so it guesses, and the guess is loud. A Custom GPT carries your rules and your policy into every chat. You stop re-explaining the firm each time, and every reminder comes out with the same facts in the same order. The stage rules also stop the two mistakes that cost the most: being so soft the client does not realize it is late, and being so harsh the client leaves.

How to use this in your bookkeeping or finance work

Swap in your own collections policy, payment options and a reminder you were happy with. Then, when you run your accounts receivable aging report, give the GPT the client's first name, invoice number, amount, due date and days overdue, and let it draft the email while you check the numbers. It fits well into a weekly AR routine. Keep a human in the loop: you read and send every reminder yourself, and you decide what happens with anything past 60 days. This lesson is about clear, respectful writing. It is not legal advice about collections.

Step by step: Build a Custom GPT That Writes Overdue Invoice Reminders

All 16 steps of the interactive module, in order. In the module you watch each one, then do it yourself in a practice copy that checks your work.

1. Ask ChatGPT

1. Click the message box. Click the message box at the bottom to start typing.

   Most people open ChatGPT and just start chatting. They type a quick prompt, get a generic answer, and wonder why AI feels underwhelming. Let's see exactly what that looks like.

2. Type a prompt. Click the Paste prompt button on the message box (or type anything close to it): Write a payment reminder to a client whose $850 invoice is 45 days overdue

   We will ask ChatGPT to write a reminder for an overdue invoice. No context, no instructions, just a bare prompt — the way most people use it.

3. Send the message. Click Send.

2. Explore GPTs

4. Open the More menu. Read the response — it is generic and full of emojis. Now click More in the sidebar.

   Look at that. All caps, emojis, threats of collections and legal action. No invoice number, no amount, no way to pay. Send that and you lose the client. That is what happens when you give AI zero context. Let's fix that.

5. Open GPTs. Click GPTs to explore Custom GPTs. In the current ChatGPT the sidebar says Explore and there is no More menu, so you can also go straight to chatgpt.com/gpts.

   A Custom GPT is your own version of ChatGPT with built-in instructions and knowledge. Instead of explaining who you are every time you chat, the GPT already knows. In the current ChatGPT that button is called Explore, or you can go straight to chatgpt dot com slash gpts.

6. Create a GPT. Click + Create to start building your Custom GPT.

   This is the GPT store. You can browse what others have built, but we are going to create our own.

3. Name your GPT

7. Click the Name field. In the real builder you start on the Create tab, which is a chat. Click the Configure tab first, then click the Name field.

   This is the GPT builder. On the left you configure it, on the right you can preview it. First, give it a name. In the real builder you start on the Create tab, which is a chat, so click the Configure tab first.

8. Name your GPT. Enter the name: “Payment Reminder Writer”

   Call it Payment Reminder Writer. This is what shows up when you open it later.

4. Add instructions

9. Click the Instructions field. Click the Instructions field.

   Now here is the important part. The instructions tell the GPT exactly how to write, what tone to use at each stage, and what it must never say.

10. Paste your instructions. Paste in the instructions to add your reminder rules.

    These instructions cover the tone, how firm to be by days overdue, what every email must include, and what the firm never threatens. This is what turns a generic chatbot into a tool that writes like your best client manager.

    You write payment reminder emails for Maple Street Bookkeeping, a small bookkeeping firm. Clients are small business owners who pay us a monthly fee. You turn a few invoice details into a reminder the owner can review and send.
    
    VOICE & TONE
    - Sound like a calm, friendly professional who values the relationship — never like a debt collector.
    - Be polite and clear. Be firmer as the invoice gets older, but never rude.
    - Use short sentences. Write at a 7th-grade reading level.
    - Never use emojis, all caps or exclamation points.
    - Never use threats, guilt or shame. Never say "final warning", "or else" or "legal action".
    
    TONE BY DAYS OVERDUE (from the Collections Policy)
    - 1 to 15 days: a friendly note. Assume they missed it.
    - 16 to 30 days: a clear reminder. Ask them to pay or reply with a date.
    - 31 to 60 days: polite and firm. Ask for payment within 7 days and offer a short call or a payment plan.
    - 61 days or more: do not write the email. Say the owner handles it personally.
    
    STRUCTURE
    - Subject line with the invoice number.
    - Greet the client by first name.
    - State the invoice number, amount, original due date and days overdue.
    - Give the ways to pay. End with one clear next step.
    - Keep every email under 150 words.
    
    CONTENT RULES
    - Only use the payment options and terms in the Collections Policy document.
    - Never invent fees or interest. Only mention a late fee if the client's engagement letter includes one.
    - Never mention collection agencies, credit reports or lawyers.
    
    FIRM CONTEXT
    - Maple Street Bookkeeping has 4 people and serves about 60 small businesses.
    - Invoices are due 30 days after the invoice date.

5. Upload knowledge

11. Upload your collections policy. Click Upload files to find your collections policy document.

    Instructions set the rules. You can also upload files — your collections policy, your payment terms, sample reminders. The GPT uses them as reference.

12. Open the collections policy. Click Maple-Street-Collections-Policy.pdf to preview it.

    Here is your Google Drive. Find the collections policy and click it to see what is inside.

13. Attach the document. Scroll through the document, then click Select to attach it.

    This is your collections policy: payment terms, the tone for each stage of overdue, what every reminder includes, and a sample. The GPT will follow it every time.

6. Test your GPT

14. Start chatting. Click the message box in the preview panel to test your GPT.

    Everything is set. We have a name, detailed instructions, and our collections policy uploaded. Now let's test it with the exact same prompt we used before.

15. Type the same prompt. Enter the same prompt: “Write a payment reminder to a client whose $850 invoice is 45 days overdue”

    Same exact prompt. Word for word. The only difference is that this GPT has our instructions and collections policy.

16. Send the message. Click Send.

    Send it and watch the difference.

Screenshots

Frequently asked questions

Can ChatGPT write payment reminder emails for overdue invoices?

Yes, but a bare prompt tends to give a generic or aggressive email. A Custom GPT with your collections policy and a few rules writes reminders that include the right facts and match the tone to how late the invoice is.

Is this lesson free?

Yes, Stepthrough is completely free. To build it for real you need your own ChatGPT account. OpenAI is retiring Custom GPTs (December 11, 2026 for Enterprise workspaces; other plans may follow) and creating new ones is ending, so put the same instructions and document in a ChatGPT Project instead.

Will the GPT send emails to my clients on its own?

No. In this lesson the GPT drafts the email and you copy it into your own email. You review and send every reminder yourself.

Does this give legal advice about debt collection?

No. The lesson teaches you to write polite, clear reminders based on your own policy. The fees and terms shown are made-up examples. For questions about late fees or collection rules, talk to a qualified professional.

How does the interactive lesson work?

Each step is shown to you first on the left (Watch). Then you do it yourself in a practice copy on the right (Your turn), and every click is checked. If you get stuck, press Do it for me. It is a practice copy, so no real accounts are touched.

Start this module free

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