Express Invoice will give you a pop-up box before sending the email. This has a place where you can add a message. I have found no way to change the default text for the rest of the email, though. My default text says this:
"This message contains Invoice ### from (my company). If you have questions about the contents of this message or Invoice, please contact (my company)."
It fills in my company name and the invoice number for the email. After that it lists my company name, address, and telephone number according to what I put into the General tab of my preferences. Then it attaches the invoice or statement below that.
As a side note, I always put a comma, (no space needed), then my email address after I put in the customer's in the customer info screen. This way I always get the same email that I sent to the customer.
Teri