public function setUpOnce()
 {
     if (!ShoppingCart_Controller::has_extension('ShoppingCartAjax')) {
         ShoppingCart_Controller::add_extension('ShoppingCartAjax');
     }
     if (!VariationForm::has_extension('ShoppingCartAjax')) {
         VariationForm::add_extension('ShoppingCartAjax');
     }
     if (!Controller::has_extension('AjaxControllerExtension')) {
         Controller::add_extension('AjaxControllerExtension');
     }
     parent::setUpOnce();
 }