When I looked it up, it shows me in my area a different clearance price that range from $13.40-$72.00. So when you pick the color you want, and the size you need the price will change for the current price for that item.
When something goes on clearance, the more popular items will sell for more because the store is able to make at least some money on it, however for those that were less popular or didn't sell at all would get marked down for less so they can get rid of more of it. When you first clicked on the listing, it gave you a range of prices, because Sears.com doesn't know which one you are yet wanting. Which is why it told me prices are between $13-$72 but when I click on my size and the color I would want, it gave me a $14.40 price.
Also, some items aren't free shipping and are only if you are buying $35 worth of stuff. So if you were buying just the one shirt, shipping would be added on and that could also explain why you are seeing more than $18. I know when I click on 3 XL the price went from $14 to $17.90 so that could be another thing as well. Since it's a much larger item, more stuff went into making it so they would cost a little more.