at line 662, the hidden form element is using the php code:
<input name="shippingOption" type="hidden" id="shippingOption" value="<?php echo((isset($_POST["shippingOption"]))?$_POST["shippingOption"]:"") ?>" />
you need to change this to use ASP code:
<input name="shippingOption" type="hidden" id="shippingOption" value="<%= Request.Form("shippingOption") %>" />