We mentioned yesterday that people search for "free shipping." There's no doubt.
And people search for products with "free shipping" as a modifier.
Guess what else they search on? Your store name plus free shipping. And who ranks? Often affiliates, deals and coupon sites.
Check out the related searches suggested by Google when you search for "free shipping."
Let's click on "free shipping JC Penney":
Now, JC Penney needs a page optimized for "free shipping" so it would rank #1. Of course, JC Penney doesn't need to offer free shipping all the time to have its own dedicated page. The page just needs to exist, all the time, as a landing page for "free shipping jc penney" traffic.
These searchers are going to find the coupons one way or the other, so why not have a landing page (perhaps a sub-section of customer service) that shows which products qualify for free shipping at any given moment, and has a link to an RSS feed for future free shipping offers, or an email sign-up link? Then you can even segment these cheapo-s out into their own bucket in your email campaigns (I'm kidding, but I'm not kidding).
And don't forget keywords in the title tag: "JC Penney Free Shipping Offers."