Greetings, @usercleanedbyclaires.

You can easily change the default email message when sending invoices to customers. I can guide you on how to do it.

1. Go to the Invoice menu.
2. Click the Create invoice button. 
3. Select the Customize option above to open the customization window.
4. Hit on the Email drop-down arrow.
5. From there, you can edit the message for your customer.
6. Click the Save and Close.

You can read through this article for more detailed steps in personalizing your invoices: Create invoices in QuickBooks Self-Employed

Let me know if you have other concerns in the comments below. I'll get back and take care of them for you.