on the checkout page, add the checkbox:
<label for="expedited">Expedited Shipping:</label>
<input name="expedited" type="checkbox" id="expedited" value="1" />
on the confirm page, near the bottom of the page, there is a collection of hidden element, add a new hidden element using the same name as the checkbox on the checkout page:
<input name="expedited" type="hidden" id="expedited" value="<?php echo((isset($_POST["expedited"]))?$_POST["expedited"]:"") ?>" />
next, add a set session value behavior. on the server behaviors panel, click the plus button and select eCart -> General -> Set Session Value.
for the trigger and value, click the lightning bolt icon and select the "expedited" form element. set the name to "expedited"
in the eCart object, create a new shipping rule.
set the trigger to based on session variable value:
if the session variable "expedited" has a value of "1"
and use a flat rate calculation of 5