Hello, Wayne
Actually, ViArt Shop already has such functionality. To attune your custom message for a certain product, please go to Administration > Products > Products&Categories, select a certain product and click the Edit link. Go to the 'Notification' tab, select under what status to send the notification and enter the information you want to send to your customers in the Messages textarea.
We would recommend you to read more about setting Notifications at:
http://www.viart.com/notification.html
With kind regards,
ViArt Support Team