How to get IAP working in Unreal Engine?

We're also investigating how to get IAP up and running with Unreal. Hope you get a good response to the question. 

I've made some slight progress with this, but one key thing I don't understand.

In the documentation it explains how to launch the checkout flow, but I can't find anything about handling the result with a callback etc.

I've found people discussing this in the Unity forums using something like this:

IAP.LaunchCheckoutFlow(temp).OnComplete(LaunchCheckoutFlowCallback);

But I can't find how to do this with the native C APIs.

How do I know if a successful purchase has been made?

I would also like any help for IAP in Unreal, specifically using blueprints-only. I have tried everything and have reached a wall. The SDK Box plugin seems hopeful, but is specially catered to iOS and Android. There is no specific IAP method for Oculus, other than "VR Monkey". I have not done too much research into this, but the developer says the code needs slight adjustments (which you can email them about) as is crashes on load for me.

I am creating custom structures for storing products, using the same product ID's which 'I've setup through the Oculus' developer company page. Then, I'm connecting it to a "make inApp Purchase" node through Unreal's blueprint. It always fails, though, when testing from a test user and the downloaded RC version of the app.