I’m really looking for this too. My site is very simple and I don’t need a ton of extras, but having a discount on the checkout page would be a huge deal. I keep trying to get customers to come back by offering them deals, but I have to go through each item and change the price by hand. It would be so much better if I could just email existing customers a discount code. At the same time, I wouldn’t be losing out on any additional revenue from first time customers.