Hello there, @grazedbyasch-gma.
Currently, you're only able to alter the default text of your sales forms (i.e. invoices, estimates, sales receipts, credit notes, statements, and refund receipts) through the Account and Settings page. However, you have the option to manually type in your message directly on the Body of the email after clicking the Receive payment. This way, you can personalise the content of your notification.
I've attached screenshots below that show the last five steps.
In QuickBooks, you're able to personalise the content of your sales transactions and include the information that matters most to your business. To learn more about customising, attaching files, and emailing forms to your customers, kindly refer to this article for the complete details: Email and customise sales form.
Additionally, you can also email forms and reports to multiple addresses. If you wish to know more about this, you can check out this article: Multiple Email Addresses. It includes steps about saving default email addresses when creating sales forms.
You're always welcome to comment below if you have other concerns or follow up inquiries about managing transactions in QBO. I'm just around to help. Take care always.