Hey there, cosmoscounsellin.
If you have sent the receive payment, the option to edit the email message is unavailable. If not, you can manually edit the information after you click the Save and send button.
Here's a sample screenshot of what it looks like:
For more information on this, please check out this detailed article: How to email and customize the sales form.
I'd be glad to help if you have other questions or concerns. Thanks.