I came here searching for an answer to the same question. I see there really is no answer. If QB created this email that cannot be changed (except to have to change it EVERY time you want to send a receipt), they need to update on their end. It is very redundant to essentially say "Thank you" twice, two lines in a row. I remove the first one every time. 

Bottom line is, I don't understand why this would not be a customizable form/email as the other financial form emails are able to be customized.