DRP Coupon is a WordPress Coupon Plugin by Direct Response Publishing which Allows You To Add Coupons that Expire To Your Posts and Pages. You can set your coupons to expire at the end of a certain date and they will disappear from your post or page. Add a regular link or an affiliate link from which the coupon will go to when clicked. A person cannot copy the code unless they click on the link. Once clicked it will copy the coupon code to the clipboard ready for easy pasting.
Plugin homepage: http://www.directresponsepublishing.com/