Edgar,
You can use multiple price definitions. 25% Off, 50% Off, etc
Using Rules & Actions, setup a 25%/50% Off Button (Ticket Tag) and have it change the Price Tag for the last item added. This will help with controlling which items get a discount.
What this does allow, is if you have a Discount Price Definition, have it selected on login, and each day you update the items that you want to discount by adding a discounted price. If an item does not have a discounted price then it is sold at the regular price.
This is more secure when you have staff.
My other solution will need Emre to add a new Action to use the Change Price button. Since we can now multiply numbers with Update Program Settings, we could setup 25% & 50% Off Actions and an Action to Change Price for Last Item Added.
Line Added To Ticket -> Save Item Price -> {:Last Item Price}
Ticket Tag Selected (50% Off) -> Multiply {:Last Item Price} by Discount (0.50 - 50% Off) -> Change Price Last Item Added
That would make an easier way to apply single item discounts. But easier for staff to apply discounts to friends.