Adding a message to the checkout email
Since Zen Cart 1.5.6, the capability to add a static message to every email has been built in. The original file is located at ‘includes/languages/english/email_extras.php’. If you edit the file, save it as
includes/languages/english/YOURTEMPLATE/email_extras.php to add it to your template’s override. If the file already exists at ‘includes/languages/english/YOURTEMPLATE/email_extras.php’, then set the defined constant
EMAIL_ORDER_MESSAGE with your additional message and save the file.
If your Zen Cart version is older than 1.5.6, you may use the Order Message plugin.