Adding a message to the admin order update email
Since Zen Cart 1.5.7, the capability to add a static message to the admin order update email has been built in. The file containing the message is located at ‘admin/includes/languages/english/email_extras.php’. Edit this file, and set the defined constant
EMAIL_ORDER_UPDATE_MESSAGE with your additional message and save the file.
There is also a notifier called
ZEN_UPDATE_ORDERS_HISTORY_SET_ORDER_UPDATE_MESSAGE which fires when the message is being added, providing the opportunity for customization.
You can test your changes to the order update email simply by updating an order on the order details page.