I'm building a custom sales channel (for my mobile app) that can be used by other merchants. I have a question regarding payment. I want it so that whenever a user on the app pays an amount for a products, a % of this price goes to me and the rest to the merchant. Let's say an user buys a €100 from a merchant through the custom sales channel on my mobile app. I want to be able to take 10% commission of this (as agreed by the merchant). So, when the user pays €100 through my app, €90 goes to the merchant and €10 goes to me.