Commissions by Cl*** (on inventory item)

I have about 20 inventory items with different parent items that I want to pay commissions on based on quantity sold (as a group), ie sell 3 or more of any of these items and earn $20 per item sold.

If I base my commission on item quantity sold this wll not work because if someone sells 1 of item a, one of item b, and one of item c they should be eligible for $60 in commission but basing on item quantity sold would not work unless they sold 3 of one particular item.

Scheduling by parent item also will not work since these items have different parent items.