# Build a Custom GPT That Writes Overdue Invoice Reminders

> Build a Custom GPT that follows your collections policy and writes polite, firm payment reminder emails for overdue invoices. Free, step by step, no code.

Source: https://learn.ynteractive.com/content/custom-gpt-payment-reminder-emails-overdue-invoices · Updated 2026-11-03 · Free from Stepthrough (https://learn.ynteractive.com)

## Practice it step by step

[Start the free practice](https://learn.ynteractive.com/modules/chatgpt-custom-gpt-finance)

[Take the free module: why it matters, practice and a quick check →](https://learn.ynteractive.com/courses/ai-for-finance/14)

You do every step yourself in a practice copy of ChatGPT. It checks your work as you go, and “Do it for me” finishes any step you get stuck on. Free, about 8 minutes, and nothing touches your real accounts. [Open full screen](https://learn.ynteractive.com/modules/chatgpt-custom-gpt-finance)

Free · Live now · [AI Automation for Finance](https://learn.ynteractive.com/courses/ai-for-finance) · [ChatGPT](https://learn.ynteractive.com/courses/chatgpt-for-everyday-work) · Google Drive · ~8 min · 16 steps · Beginner

Updated November 3, 2026. Built against the tool’s current screens and checked step by step. [How we check this lesson](https://learn.ynteractive.com/content/about#how-we-check)

**OpenAI is retiring custom GPTs on December 11, 2026 and replacing them with plugins.** [Here’s how to save yours →](https://learn.ynteractive.com/content/explainers/chatgpt-course-for-work#save-your-custom-gpt-before-december-11)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](https://learn.chatgpt.com/docs/migrate-custom-gpts).

**Custom GPTs are being retired;** see [ChatGPT Projects vs Gemini Gems vs Gemini Notebook: Which to Use](https://learn.ynteractive.com/content/chatgpt-projects-vs-gemini-gems-vs-notebooklm). This lesson stays here for reference, but it is no longer in our lesson lists.

Ask plain [ChatGPT](https://learn.ynteractive.com/courses/chatgpt-for-everyday-work) for an overdue [invoice](https://learn.ynteractive.com/content/glossary#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](https://learn.ynteractive.com/content/glossary/custom-gpt) that follows the firm's [collections policy](https://learn.ynteractive.com/content/glossary) 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.

## At a glance

| Question | Answer |
| --- | --- |
| What you build | See why generic prompts give generic results, then build a Custom GPT that follows your collections policy and writes polite, firm reminder emails for overdue invoices. |
| Tools | ChatGPT |
| Time | About 8 minutes, 16 steps in 6 parts |
| Level | Beginner, no coding needed |
| Cost | Free |
| Ways to learn it | [Practice it step by step](https://learn.ynteractive.com/modules/chatgpt-custom-gpt-finance) |

### The 6 parts, in short

1. Ask ChatGPT: Click the message box, Type a prompt, Send the message.
2. Explore GPTs: Open the More menu, Open GPTs, Create a GPT.
3. Name your GPT: Click the Name field, Name your GPT.
4. Add instructions: Click the Instructions field, Paste your instructions.
5. Upload knowledge: Upload your collections policy, Open the collections policy, Attach the document.
6. Test your GPT: Start chatting, Type the same prompt, Send the message.

**Start here first:** [Get On-Brand Social Posts Every Time using a ChatGPT Custom GPT](https://learn.ynteractive.com/content/custom-gpt-social-media-assistant). 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.

## Try it yourself, free

Reading the steps is a start. Doing them is how it sticks. The interactive module walks you through every click in a practice copy of ChatGPT, checks each step, and never touches your real accounts.

[Start “Build a Custom GPT That Writes Overdue Invoice Reminders”](https://learn.ynteractive.com/modules/chatgpt-custom-gpt-finance)

## 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.
    ```text
    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.

## 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](https://learn.ynteractive.com/modules/chatgpt-custom-gpt-finance)

- Next in AI Automation for Finance: [Build an AI Agent That Categorizes Business Expenses](https://learn.ynteractive.com/content/n8n-ai-agent-categorize-business-expenses-google-sheets)

## More in AI Automation for Finance

- [Build an Expense Reimbursement Form With AI Policy Checks](https://learn.ynteractive.com/content/expense-reimbursement-form-ai-approval-lovable-n8n): Build an expense reimbursement form in Lovable for Maple Street Bookkeeping and connect it to n8n — the AI agent checks each request against the policy, saves it to…
- [Invoices from Your Inbox to Google Sheets, with Approval](https://learn.ynteractive.com/content/invoice-inbox-to-google-sheets-approval-n8n): Build an n8n workflow that reads each vendor invoice PDF from Gmail with AI, logs the vendor, invoice number and total in Google Sheets, and emails the owner an Approve…
- [Docparser: Pull Invoice Data from PDFs into a Spreadsheet](https://learn.ynteractive.com/content/docparser-extract-invoice-data-pdf-to-spreadsheet): A business case for bookkeepers and office managers who retype vendor invoices every week: have Docparser pull the vendor, invoice number, date and total out of each…

## More ChatGPT tutorials

- [Prompt Writing 101: Role, Context, Task, Format](https://learn.ynteractive.com/content/prompt-writing-101-role-context-task-format): Ask ChatGPT for the same customer email twice, once vaguely and once with a four-part prompt, and see exactly why structure gets a better answer.
- [Send a Same-Day Follow-up After Every Discovery Call using ChatGPT](https://learn.ynteractive.com/content/chatgpt-discovery-call-follow-up-email-sales): Turn a discovery call transcript into the decisions, the next steps with owners and dates, deal notes for your CRM, and a follow-up email the prospect gets the same…
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- [Get Your Business Recommended by ChatGPT](https://learn.ynteractive.com/content/get-recommended-by-chatgpt): Ask ChatGPT who it recommends in your town, see which pages it trusts, then use it to write an FAQ page and Business Profile copy that answer real customer questions.
- [Run a Weekly Ops Review Where Every Task Has an Owner using ChatGPT](https://learn.ynteractive.com/content/chatgpt-weekly-operations-meeting-recap): Turn a weekly operations and vendor review into the decisions, the next steps with owners and dates, the risks to watch and a team recap, without turning a vendor's…
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## Same business, other tools

Maple Street Bookkeeping shows up in these modules too, built with different tools.

- [n8n: Execute Step, Execute Workflow and Publish](https://learn.ynteractive.com/content/n8n-execute-step-execute-workflow-publish): Learn the buttons in today's n8n editor: run one node with Execute step, pin test data, run everything with Execute workflow, then Publish and copy the Production URL.

## From the blog


[All blog posts](https://learn.ynteractive.com/content/blog)

## Terms in this lesson

Plain-English definitions, each with an example from a free lesson.

[Accounts receivable turnover](https://learn.ynteractive.com/content/glossary) · [Bad debt](https://learn.ynteractive.com/content/glossary) · [Cash application](https://learn.ynteractive.com/content/glossary#cash-application) · [Collections](https://learn.ynteractive.com/content/glossary) · [Custom GPT](https://learn.ynteractive.com/content/glossary/custom-gpt) · [Days sales outstanding (DSO)](https://learn.ynteractive.com/content/glossary#days-sales-outstanding) · [Dunning](https://learn.ynteractive.com/content/glossary#dunning) · [Early payment discount](https://learn.ynteractive.com/content/glossary)

[The full AI glossary](https://learn.ynteractive.com/content/glossary)

## Related prompts

- [Final notice for an overdue invoice prompt](https://learn.ynteractive.com/content/custom-gpt-payment-reminder-emails-overdue-invoices): Draft a firm, respectful final notice on a seriously overdue invoice that states only the next step in your own policy. No…
- [Payment reminder email prompt](https://learn.ynteractive.com/content/custom-gpt-payment-reminder-emails-overdue-invoices): A four-part prompt for a friendly first reminder on a late invoice: invoice number, amount, due date and ways to pay, with no…

[All free prompt templates](https://learn.ynteractive.com/content/prompts)

## More terms on this topic

[Accounts receivable vs accounts payable](https://learn.ynteractive.com/content/glossary) · [Invoice](https://learn.ynteractive.com/content/glossary#invoice) · [Late fee](https://learn.ynteractive.com/content/glossary#late-fee) · [Lockbox](https://learn.ynteractive.com/content/glossary#lockbox) · [Order-to-cash (O2C)](https://learn.ynteractive.com/content/glossary)

## More prompts on this topic


## Browse related tutorials

[AI for Finance (4)](https://learn.ynteractive.com/courses/ai-for-finance) · [ChatGPT tutorials (65)](https://learn.ynteractive.com/courses/chatgpt-for-everyday-work)