How do I install a pay wall with IAP?

I want to install a paywall with IAP. Is there an IAP tutorial? I’m not sure exactly how to use the API. Has anyone done something similar to this? The game will be free, but I want a pop-up after level 4 asking them if they would like to continue to please pay .99 cents. [import]uid: 75779 topic_id: 36866 reply_id: 336866[/import]