This slick addon to Zen Cart gives you a drop down box where  you can update an order's status and it automagically saves the status update and records it in your comments section of your order. 

NOTE:  It does NOT send an email to the customer about the update.

Zen Cart Version 1.5.0, 1.5.1, 1.5.3, 1.5.4, 1.5.5