craft()->on( 'commerce_orders.onSaveOrder', function ( Event $event ) {

        $order = $event->params['order'];

        if(craft()->request->getPost('validateShipping')){

            // add error to shippingMethod and see if we can get it
            $order->addError('shippingMethod',Craft::t('Please select a shipping option.'));
            $event->performAction = false;

            // pass a flash for testing
            craft()->userSession->setFlash('error', 'No shipping elected');
        }
    });