It’s %5 no mater where you sell that goes to epic after the 1 mil dollar mark (gross revenue). The other charge you pay from selling on stores is separate. I’m not aware of any paperwork you have to fill out regardless of what game you make, where you sell it, or how much you make from it. This seems like something we all need to know eventually. I have not released a game yet but I’m learning about this thanks to your question. Again this is what I think from my understanding of what I read. It might be best to contact Epic directly to get some specific answers. If you do or if you see that anything I said was wrong, please reply about it.