how to add weight field on checkout page in Jigoshop Plugin

Simply use a callback that you hook to wp_mail(). The first line of the function:

extract( apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) )

The third argument $message is what you want to alter. Just return an array in the callback where you merge the original with your alterations.