If you're doing it from your iPhone it's easier, Click on my Listia, Open, Click on the picture, Click on edit on the top right corner Then click on delivery type in zero for the cost.
When you do it on your iPhone you always have to re-update location for some reason, After that just update the listing and it should work.
If you're doing this from your computer, once you log on. You will see a list under " my activity" In orange letters you will see " my open auctions" next to that in blue you should see your current listing for the
Xbox 360 games with kinect "click on it"
Once it opens right above your picture of the listing
You see 3 icons a pencil, edit and a red X
Click on edit
You will see pictures, description, delivery then US shipping right next to it shows the type you should be able to click on it and change It to free.