Cool. The actions are here:
https://github.com/dumbgamedev/IAP_PlaymakerIt requires that IAP be installed via unity. You need this installed before the actions, or Unity will throw a bunch of compiler errors.
Open the services window in Unity and signup/sign in as necessary. Then choose to enable In-App Purchasing and follow the steps.
Store products are added using the Unity IAP window.
Window > Unity IAP > IAP Catalog.
The package includes a test store action, which will spawn a gameobject with a script attached that works like the store for simple testing. Note however, it does not simulate "Restore Products".