Discount problem

Hi ? We allow for 15% off any mix of 12 items or more from one department. The problem is that if we apply a 15% discount to the entire order, it would then discount items from other departments and sale items from that department as well, which are not normally discounted. I currently got around the problem by setting all the other departments and sale items to ?non discountable'. The problem is, that, at times, I need to discount or adjust those other items as well. Is there any way to identify only those items that the discount would apply to, so I don't need to set every other department to nondiscountable ? I saw a few posts that show how to write C or VB code and run it from within the POS - would that solve my problem ? Thanks very much !