Hello there, Drhowardy.
As of the moment, we're unable to save the information added in the message box. The same as the email message when you send the invoices. You'll have to manually change the message every time you create an invoice, and send it.
Your idea is great, though. I'll surely share this to our management team so they can pass this along to our product engineers. This will be reviewed and might be added in future updates.
In case something arises, please let us know. We're just around.
