The data from the custom fields is made available to the developer of the game on the dashboard and that’s it. You are responsible for deciding what to do with it and where to put it. Hope that helps, tell me if you have any questions.
Is it possible to have it so that the buyer can purchase a customisation for their game alone?
I can’t think of an easy way to do this right now. We don’t support custom builds of a game, so if something like this was supported it would probably have to be done via some kind of API call, which would require an internet connection for the person to get access when playing the game. In any case, we don’t make reward input fields available anywhere outside of your dashboard, so you would still have to manually do this.