Hi there, drncmd.
With QuickBooks Self-Employed, all changes made will be applied to all invoices. We'll have to edit the message for each customer manually.
You can do that by clicking the Email dop-down of the Invoice form. Then, enter a Message.
In case you want to let your customers pay the invoices online, we can send an online invoice: Take and Process Payments with QuickBooks Payments.
I'm just a post-away if there's anything that I can help. Keep safe!