add ability for customers to use a code to get discount on my ecommerce site

i have an ecommerce website ([login to view URL]) and would like to be able to give new and returning customers a code (that i can change often at my will and to be able to give out several different ones at one time)so they can get a discount on their total purchase (ie: $5 off for the month of June - they get a letter or number code to enter in order to receive) I'd like to be able to set up multiple coupon codes with percentage or absolute amount discount. I'd like to be able to add/edit/delete and set expiration dates of coupons. So on the site a coupon code field will be there and once customer fills in the coupon code, the amount will be deducted at checkout.