Possibly in future editions or updates your team could rework the coding so if the cashier selects a Discount (ie.: a Product Item in which the selected field is "Coupon%" or "Coupon") then the system will only allow them to select 1 or the other, basically only 1 Discount can be applied per purchase. If they try and select 25% OFF discount more than once it will not allow it. Isn't that technically possible to do? It would alleviate any loop holes with making products FREE or applying a certain percantage of discount multiple times.
I can't see where this would have any negative effect since it is not discounting a single item, but it discounts the entire purchase. You would only ever want to discount the ENTIRE purchase 1 time anyway. Anything more technical could be adjusted by a GM with security clearance to do so.
Just an idea that would be helpful for on the consumer end.