Hello there, karmaprojects.

Please know that an option to manually compose a customer letter in QuickBooks Online is currently unavailable. However, you can consider adding the customer letter as an attachment when creating an invoice in the program as an alternative. Let me provide the steps to ensure you can complete the process.

First off, you have to make sure to use the supported attachment file types in QBO. Then, you can start following the instructions below.

1. Open the Invoice page.
2. Select Add customer and choose a specific person from the dropdown.
3. Review the Invoice date, Due date, and Terms. Enter new dates or terms, if you need to. 
4. Select Add product or service and select a product or service from the dropdown arrow.
5. Go to the Attachments box and add the customer letter.
6. Fill in the other needed fields.
7. When you’re done, click Save and close.

Moreover, you can also have the option to a send reminder message to your customer. You can open this article on how to perform the process: Send invoice reminders automatically or manually in QuickBooks Online.

In case you need steps on how you can view all the status of sales transactions like open and paid invoices, you can read this article: View sales transactions in QuickBooks Online.

If you have any other follow-up questions about the customer data in QuickBooks, please add a comment below. We are right here to help, karmaprojects. Have a great day!