You can try adding the form data binding and select the checkout page to see if that updates the bindings and allows the select list to be available to you. If that does not work you can set the trigger manually by selecting any posted form element and changing the name listed in the trigger like this:
php:
<?php echo isset($_POST['your form element'])?$_POST['your form element']:"" ?>
This is just to get at the value, the actual trigger should look like this when you see it in the code:
php:
if((isset($_POST['your form element'])?$_POST['your form element']:"") == 'UPS'){
the UPS shipping quote server behavior here
} else {
the USPS shipping server behavior here
}
If you post back with the code for the select list that is in your checkout form I can give you a better example of this.
