We are currently running a WordPress site with full version MarketPress, in which it allows users create their own sub-sites. On these sub-sites users can post their digital products for sell. There is a built-in function in MarketPress that the admin can set in WordPress Dashboard, allowing a certain percentage of fee (eg. 15% of the profit) from all sales in all sub-sites goes to the main site admin's paypal account.
What we want to achieve is to allow certain sub-sites bypass this fee, while 15% of all other sub-sites' sales goes to the site admin.