We would like to be able to use a coupon code in the discount field on an invoice so we can track the discount. Right now you can enter an amount but no description for the discount nor the link to an existing coupon code. If I want to track all the discounts given under a certain "coupon code" I can not do this. Plus can't tell "why" I gave a customer a discount on a past invoice. Seems like it would be a very simple fix.