Only way I know of is to set up price codes for the items and use Price Code commissions.

Price codes are part number specific.  So price code 1 would apply to the entire order but for some items Price Code 1 gets 5% commission and for others it gets 10%.  You can manually enter the commissions by line item as well if you set the Commission Default to enter Commissions at line item entry but that puts the responsibility on Order Entry to do it right.