Hi,
since you can only consume web services (the source of action is always external), you would need a small program that does the polling for you. We have made such a program (very small .net c# code) to have a "worker process" for post-processing data which has been delivered by other interfaces. It is in the task scheduler of the host and starts every 5 minutes. Sounds like low-tech, but it's very reliable.
In your case, such a program might be called periodically and updates the order status info in the ecommerce store database. For a real event-driven callout, you would need to do things with either linked objects (link the order status field to a table/field in NAV) or .net interop.
with best regards
Jens