Adding AJAX to drupal commerce checkout pane
I think it might have to be related with the classes on the checkout form, if you check the commerce shipping module - it has a good ajax example on replacements - you can see how it does:

// Add a radios element to let the customer select a shipping service.
$pane_form['shipping_service'] = array(
  '#type' =&gt; 'radios',
  '#options' =&gt; $options,
  '#ajax' =&gt; array(
    'callback' =&gt; 'commerce_shipping_pane_service_details_refresh',
    'wrapper' =&gt; 'commerce-shipping-service-details',
  ),
);


And then it forces the class with a prefix:

$pane_form['service_details']['#prefix'] = '&lt;div id="commerce-shipping-service-details"&gt;';
$pane_form['service_details']['#suffix'] = '&lt;/div&gt;';


The ajax callback is the usual one:

/**
 * Ajax callback: Returns the shipping details form elements that match the
 * currently selected shipping service.
 */
function commerce_shipping_pane_service_details_refresh($form, $form_state) {
  return $form['commerce_shipping']['service_details'];
}


So definitely possible, check the css classes you're dealing with.