Hi jd, HK have been running this promotion since late November last year... Every now and then, at random and usually unpredictable times, HK runs a freebie. To see the freebie, you need to be logged in and have something in your cart that's more than about $15 and less and $100. You'll get a popup that tells you what the freebie is. You click on the link (to put the product in your history), fill up your cart with >$100 worth of stuff, and THEN add the freebie item. In your cart, it should come up as $0.00. If not, you've not been quick enough.
Note that you still have to pay shipping on the freebie. Given that they're often planes, the shipping can be pretty high. Still, if you can land a good enough freebie (pardon the pun), it's pretty cool. Just be careful not to add too much crap to the order to make up the $100!