Thanks for getting back here, @PBB22.

Having personalized email messages in different invoice templates is a big help to widen your business connection with your customer.

However, as discussed in the previous replies in this thread, when you edit the email message on one invoice template it will also change the other ones you've created. As a workaround, you can create Word documents with each containing a different email message and import them to QuickBooks Online (QBO). 

Also, here is the link you can open to guide you in customizing your invoice templates in QBO: Customize invoices, estimates, and sales receipts in QuickBooks Online. This also provides you a video presentation through the entire process. 

You may also visit our QuickBooks Online Feature Requests site to track product feedbacks submitted for the program. From here, you can add your vote to the feature you need to be added. The more vote the requests gets, the higher the chance it will be added to QBO. 

If you need further assistance customizing your email messages, please post a reply below. I'm always around to lend you a hand. Have a good one.