The Ultimate In-Apps API based on iOS Native and Android Native plugins implementation. You can also use API directly inside Unity editor, in this case, editor API implementation will emulate real server workflow. So you can test the implementation before building on a real device.
Before you start, just enable API for the platforms you are going to use it with. For example, if you need to use it on Android you need to:
- Open Services Page
- Click in In-App Purchases item
- Click on Android (Vending) platform
- Enable Vending on Android, and fill in your products list + additional setting required for a platform.
- Also, check out the setup guides. You do not really need the scripting guides, but you still need to do a setup inside the Google Developer Console.
You'll find more detailed description about how to use the plugin Editor UI here.
Note: The Ultimate Mobile InApps API allows you to use native InApp services through a simplified cross-platform interface. But there is no obligation to use it, you can also use native plugins API directly for each platfrom. Ot if for example cross-platform interface doesn't have platform specific feature you looking for, but a native plugin does. You can do a combination when the most part of a flow is used via Ultimate API, and a platfrom spesifi feature is used from the native plugin directly. Transaction Validation is a very good example of what I am trying to say.
The iOS and any other platform that might be added in future will have a similar approach. Once all the settings in place you can start using the In-Apps API.
I would also recommend you take a look on a native plugin's documentation as well. Since setting up process for platforms is described for each native plugin inside the plugin documentation. Plus you might find a non-cross-platform feature you would like to use.