It's my pleasure to help you change the email message in QuickBooks, @itsupport-genius.

You can change the email message to a Plain text option in the Additional email Options for Invoices section from the Account and Settings page. Plain text emails are often preferred for their simplicity and compatibility. Let me guide you on how: 

1. Go to the Gear icon.
2. Select Account and Settings.
3. Go to the Sales menu.
4.  On the Online delivery section open its content.
5. In the Additional email options for invoices field, select the  Plain text option.
6. Click the Save and Done button.

Also, I'll be adding these links that will guide you with your sales transactions and how to personalise your sales forms:

• Send invoice reminders automatically or manually in QuickBooks Online
• Customise invoices, estimates, and sales receipts in QuickBooks Online

Don't hesitate to post again if you have other concerns or if you need more help with the email messages. I'm always around to provide the information that you need.