Discount Codes in eShop

Hi I am wondering if there is something I can add or change so that i can do a discount code for 10$ off of 20$ purchase? I am looking in to doing a living social deal and need to be able to offer this discount if not possible what pluggin is easy to switch to in order to do this? I already put in so much time to eShop and I’m very happy with it other than not being able to take money off of your cart total! Thanks