When many stores learn that you will charge them 10% commission to be on the system, they increase their prices by 10%. So a Pizza that costs $100 will be sold for $110 on the app so that they can still get their normal $100 and app gets $10. Seems like a simple thing to do.
BUT putting the price of $110 on the item, means the store will only get $99 after 10% commission because the systems calculates based on the price placed on the item. This means that for the store get $100, this will have to be recalculated and will not be 10% commission anymore.
By so saying, I would request for an option to a fixed commission as well. Like on the example above, admin can just set a fixed commission of $10 on the Pizza and then store will happily receive their $100.