We've implemented an API connection with Amazon Seller Central to capture orders from Amazon and integrate them into Dynamics GP using econnect as the preferred method. All orders are created automatically in GP against the same Cust ID.
The integration doesn't just go one way. You can also control your product feed from k-eCommerce out to Amazon through the same integration. You can designate which products are sold on Amazon and drive the pricing and inventory based on Dynamics GP. Tracking information added to processed orders will also flow back to Amazon to alert the end users of shipping progress.Hope this helps!