Cannot find the Marketpress setting to take a percentage of each site's orders

I'm in the Network Settings area, looking at this page:

I cannot find anything in the MarketPress Network Options which will allow me to input a percentage to charge each one of my users (sub-directory stores like /user/store/) for each order that their customers make.

I'm trying to do this (from the MP product description):
"...use it to start your own Etsy or Ebay style network of stores… where you take a % on each sale!"

I have enabled Global shopping cart and have Paypal selected as the gateway (with my API credentials entered of course). I also have BuddyPress and ProSites installed.

Can anyone direct me to the right spot? Thank you!