On the confirm page on line 7 change:
$WAFV_Redirect = "checkout_NFC.php";
to:
$WAFV_Redirect = "checkout_NFC.php?invalid";
Then on the checkout page, you can populate the values conditionally from the failed validation when necessary. So the hidden value:
<input name="item_name" type="hidden" value="<?php echo((isset($_POST["item_name"]))?$_POST["item_name"]:"") ?>" />
$WAFV_Redirect = "checkout_NFC.php?invalid";
becomes:
<input name="item_name" type="hidden" value="<?php echo((isset($_GET["invalid"]))?ValidatedField("confirmNFC_392","item_name"):$_POST["item_name"]) ?>" />
$WAFV_Redirect = "checkout_NFC.php?invalid";