the checkout forms action is set to post to the confirm page:
<form name="ecart_checkout_form" action="confirm.php" method="post">
all form processing must occur on the forms action page
the work flow when using Server Validation when a form posts to the other page is:
On the forms action page, open the bindings panel, click the plus button and select Form Data. In the form data dialog click the folder icon and select the form page.
This adds dynamic data bindings to the bindings panel for the form.
now add the Server validation server behavior to the action page, make sure to set the failed redirect to go back to the form page.
back on the form page, add the error messages to show which fields have failed validation. Select the error message in design view, then add a Validation Show If Behavior (WebAssist -> Forms -> Validation -> Validation Show If)
in the validation show if, set the validation page to the forms action page, from the list of validations, select the validation that should trigger the error.
now select the form element, in the property inspector, click the lightning bolt next to initial value, expand the validated entries group and select the corresponding binding.