WP Simple Shopping Cart plugin allows you to configure unlimited discount coupon codes. A user will be able to apply a coupon code to get a discount for the items they are purchasing from your site.
Once you have installed the plugin, Go to following interface menu to create a new discount coupons:
Simple Cart -> Coupons
You can then give these coupon codes to your customers.
You can leave the “Expiry Date” field empty to create coupons that never expire.