You've come to the right place for help, @Tiphereth-Straker. After your customer has settled the invoice completely, you will need to manually generate and send them a receipt, as QuickBooks does not automate this process. I'm here to guide you through it.
You can follow these steps:
- Click the + New button and select Receive payment.

- Choose your customer from the drop-down menu to access their invoices.
- Check the boxes for the invoices you wish to generate a receipt for.
- Review the transaction details, then Save and send.

- Enter your customer’s email address, and feel free to personalize the Subject and Body.
- To view the receipt, click the arrow-down icon.
In addition to this, if you need to inform your customer about any outstanding balances or overdue invoices, you can refer to this resource: Create and send customer statements in QuickBooks Online.
I'm delighted to provide the help you need, @Tiphereth-Straker. If you have any more questions or need further clarification on how to send receipts to your customers, please reach out in the comments below. I’m here to assist you. Wishing you a wonderful day!