Usinng Amazon API to mark orders as shipped?

Question

I'm trying to use the Amazon Merchant API (I think that's the name of it, I can't find docs anywhere...) to mark orders as shipped. We process orders in an internal system, and I need a way to find the corresponding amazon order, and confirm shipment -- basically the same thing you do manually from the orders tab of your seller account.

I did find some sample code, but it's quite dated, and doesn't seem to work. The API returns no data or errors. Does anyone know the correct API I should be using? The sample code I'm using is the PHP sample from here:

https://payments.amazon.com/sdui/sdui/helpTab/Checkout-by-Amazon/Technical-Resources/Code-Samples-for-Working-with-Your-Cart-and-Orders