Mass action discount to show on invoice?

Hi,

The discount information is not stored with the order. The coupon information is (coupons are a type of discounts, so I just say this in case you're actually talking about coupons).
So there is no easy way to display the discounts received for an order.
With what is there now, it would require adding custom fields of the table "item" to store the information in the database and display it in the invoices/order details page, etc, and a small plugin which would gather the discount information and store it in that or these custom fields when the order is created (with the onAfterOrderCreate trigger).
So if you know PHP, it's possible to add it.