add a pickup checkbox to the checkout page:
<input type="checkbox" name="pickup" id="pickup" value="1" />
on the confirm page, there is a collection of hidden form elements, add another one using the same name as the checkbox to capture the value from the checkout page:
<input type="hidden" name="pickup" id="pickup" value="<?php echo((isset($_POST["pickup"]))?$_POST["pickup"]:"") ?>" />
then add a set session value behavior. For the server behaviors panel, click the plus button and select eCart -> General -> Set Session Value.
For both the trigger and the value, click the lightning bolt icon and select the pickup form element.
set the name to pickup
in your shipping rules, add another trigger:
Based on session Variable value
if the Session Variable "pickup" is "!=" a value of "1"