Synchroteam allows you customize the subject and body email sent along with your invoices/quotes/credit notes.
Go to (1) Settings, click on More (2) then click on the Email for Invoices (3).
To add a field to your email template, simply click on the arrow to the left of the field.
The field then moves to the selected section.
In the invoice preview, when you hit the "Send as PDF" button, the email sent out along with the invoice PDF and follows the structure in your email template.
Email example :
