Their SDK's doesn't sound like human-written SDKs, I guess these might have been auto-generated.

Disclaimer: I have worked at Amazon and have known some of their internal systems. So very much likely above is the case ;)

>$request = new MarketplaceWebServiceOrders_Model_ListOrdersRequest();

The underscores indicate that this was written (or generated) at a time where real namespaces were not an option.

Seems like the API is fine. The documentation and PHP implementation are your real complaints here. Amazon docs are always utter garbage, so no surprise there.

If it bothers you so much, get a 3rd-party lib and consume the web API directly.

The real title should be, "How not to support an API".