Amazon MWS?

I have a Zen Cart site where I sell products, and I also sell the same products on Amazon. Currently, I manually enter orders received in Amazon into Zen Cart so that they can be processed the same as all my other orders and decrement my stock counts properly. On a daily basis, I create an inventory stock file from the Zen site and upload it to Amazon so that it has the proper quantities. I would like to automate all this using the Amazon Marketplace Web Service (MWS). The approach would be to have an automated task that runs every 15 minutes use MWS to retrieve any new Amazon orders, add them as orders into Zen Cart, and then refresh the quantities in Amazon to match what they are in Zen Cart.

Does anyone have any experience using MWS, or does anyone know of any add-ons (free or otherwise) that perform functions similar to this?

Thanks ... Bowen