Alert message after add product
you need to use a callback in your submission .. something like this:

        YOURMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if($form_id == 'YourFormID') {
          $form['#submit'][] = 'custom_submit_function';
        }
}

        function custom_submit_function($form, &amp;$form_state) {
          drupal_set_message('ALERT MSG HERE');
        }


this drupal_set_message() would give you the normal drupal message

if you don't know how to know your form ID ,, just print $form_id inside hook_form_alter() you'll see it there so you can make your if condition depending on it.