I have had a situation come up a few times where it would be really nice to email a coupon to users or to put ads on the site that when clicked, will add a coupon parameter to the url or to a cookie/session or something so that when the customer completes their item selections and are checking out, the coupon is already included rather than them having to enter it once they find the coupon code to use.
I fear that users fill their carts and then go looking for the latest offer that we have sent so that they can get the discount. If when they click on an offer link (on the site or in an email) and the coupon code is in the link, it will streamline the checkout quite a bit and am sure it will make them happy. I know I appreciate it when sites like godaddy remember the offer they sent me when I am checking out.
Hope this makes sense

Thanks,
Kevin