In App purchases are their own beasts and are handled by the platform you are aiming for.
In other words, you create them in a web interface (either an Apple website for iOS or a Google website for Android) and the platform handles them.
You don't "save them", you only check in your code if your user has purchased the iAP items you created or not.
Pretty much the same for Game Center and LeaderBoards, what seems complicated is that you have to go through external services that are originally designed for coders and programmers.
Those interfaces weren't designed with the idea that non-tech savy people were going to use it, as C2 is.
But in the end, it is not that complicated takes a bit of determination to go through though.