Doesn't require our web service (Order Status System) to post status changes to 3dCart. The only requirement is that you match status names (e.g. "Shipped") between OM and 3dCart. You can actually have the system change statuses based on the events in "Status Events" w/o being an OSS subscriber, though you lose the main benefit of OSS - ie, provide any status label you like to any order from any sales channel, be it 3dCart, a manual order, or eBay/Amazon/etc, and give the customer a customizable status display page.
Synchronizing to 3dCart is provided as a convenience for shoppers that would rather return to the original order site and use 3dCart's existing status tracking system, and this is largely a matter of preference by the merchant.
Hope that helps,
Mark Setzer
Stone Edge Technologies, Inc.
http://www.stoneedge.com