Shipping Type Selection

I want to have our customers select which shipping method to post to USPS. I found the following information in another thread:

To offer a shipping type selection, you will need to add a select list to the checkout page:

PHP Code:
<select name="USPS_Service" id="USPS_Service">
<option value="FIRST CLASS">First-Class</option>
<option value="EXPRESS">Express</option>
<option value="PRIORITY">Priority</option>
<option value="PARCEL">Parcel</option>
</select>

and add a hidfden form element of the same name to the form on the confirm page that will capture the value posted:
PHP Code:
<input type="hidden" name="USPS_Service" id="USPS_Service" value="<?php echo(isset($_POST['USPS_Service'])?$_POST['USPS_Service']:""); ?>" />

next add a set session value behavior, set the trigger to any form post, set the name to USPS_Service for the value, click the lightning bolt icon and select the USPS_Service form element.

in the shipping behavior, bind the shipping service to the USPS_Service session variable.
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I was able to make the form modifications to checkout.php and confirm.php. What form do I add the session value behavior to? I assume it would go in the confirm.php but the shipping has already been calculated when the confirm page is posted.

Is it the session value that I then bind in the shipping behavior?

Thanks