The "Invoice Message" is the same every time ("Thankyou for your business") and I keep having to type out what I'd prefer. Can the Invoice Message be changed -permanently- ?

Thanks for taking the time in reaching out to us here, saspin74.

Changing the default invoice message is unavailable. For the time being, you will have to manually change the message every time you send an invoice to your customer.

To learn more about our product enhancements, you can visit our QuickBooks Blog.

I'm also adding this article as a guide: Create invoices in QuickBooks Self-Employed. 

Please feel free to leave a comment below if you have any additional questions or other concerns with QuickBooks Self-Employed. Take care and stay safe.