function buy_art_callback($ajax) {
  if ($ajax) {
    ctools_include('ajax');
    ctools_include('modal');

    $form_state = array(
      'ajax' => TRUE,
      'title' => t('Buy now with 1-Click'),
    );

    // Use ctools to generate ajax instructions for the browser to create
    // a form in a modal popup.
    $output = ctools_modal_form_wrapper('buy_art_buy_one_click_form', $form_state);

    // If the form has been submitted, there may be additional instructions
    // such as dismissing the modal popup.
    if (!empty($form_state['ajax_commands'])) {
      $output = $form_state['ajax_commands'];
    }

    // Return the ajax instructions to the browser via ajax_render().
    print ajax_render($output);
    drupal_exit();
  }
  else {
    return drupal_get_form('buy_art_buy_one_click_form');
  }
}