Thanks for reaching out to us here in the Community, @info184. 

Allow me to share some information about updating your message on your invoice template. 

When editing a message on a specific invoice template, it will automatically update the message on your other templates. Currently, there isn't an option to have the unique message and you'll need to manually edit it on each invoice. Please see the screenshot below to serve as your visual guide. 

If you're ready to send the invoice to your customer's email, you'll need to review the invoice's body before sending it. This way, you'll have the unique message for your customer (see the screenshot below). 

For more information about customising invoices, here's a great article you can refer to: Use and Customise Sales Forms. 

This will get you in the right direction, @info184. 

I'm here anytime you have other concerns. Have a good day.