Implementing delivery of purchasable content to a game

I'm writing a game and I'd like the user to be able to purchase additional levels from within the game.

Now, I could implement my own approach to delivering the content (which in my case is made up of images and xml files and so AssetBundles probably are a little heavy weight) and the purchases to the mobile app, but I'd rather follow a common pattern or make use of existing libraries.

So, how is this best achieved in Unity (note I'm using the free version)?