Yes, you can do that...but not without buying a calculation add-on like FormCalc SST for QuickBooks, or CCRQInvoice. (Google to find their Web sites.)

The two products work differently, but I believe both would support having a Discount column so that the discount percentage can be different on each Item line if you wish.  They should also let you do away with the need to have a subtotal line(s) for discounting blocks of Items.