first, add a checkbox for air shipping to the checkout page:
<input name="airShip" type="checkbox" id="airShip" value="1" />
next, on the confirm page, add a new hidden form element to the collection that already exists near the confirm button, use the same name as the cehckbox and capture the value from the cehckout page:
<input name="airShip" type="hidden" id="airShip" value="<?php echo((isset($_POST["airShip"]))?$_POST["airShip"]:"") ?>" />
then also on the confim page, add a set session value behavior, on the server behaviors tab, click the plus button and select eCart -> General -> Set Session Value.
Select Any form, post for the trigger, set the name to "airShip", clickc the lightning bolt next to value and select the airShip form element.
to create the airShip shipping rule, set the trigger to:
If Session Variable "airShip" Exists
Based on session Variable value
If the session variable "airShip" has a value of "1"