Hello there, Joelashton14.

I can see the importance of changing the email message to the invoice. We can manually change it every time you send an invoice to your customer. I see you'll be putting effort and time to make it, but this helps us change the default one.

We don't have a specific time when this feature will be available. For the time being, I recommend checking this link to learn more about our product enhancements: UK Blog. Lastly, you can check the articles below  for additional information about managing invoices and categorising transactions in QBSE: 
 

• Set up your invoice preferences. 
• Learn how to keep your transactions organized. 

Comment below if you need anything else. Stay safe.