Micro Transactions / Game Expansions

I am not sure where to post this so I will start here, what I am looking for instructions on, is how to build packages that people can purchase as game expansions, and setup the micro transactions in the Mac store, plus DLC for Steam, I was hoping to purchase some example project or find some video reference material on how to accomplish this with Unity, my game is production ready, but this will be the first game I am doing that will have micro transactions and DLC's so trying to figure out where to begin and how to do this. I know Unity games have been released that are on the App Store already, but I have no idea how they did it, STEAM so far is a blooming nightmare in general, but I figured I would start with the App Store and go from there, so any references or pointers would be very much appreciated, I am currently at a loss as to how to accomplish this task.