add html to fieldset
Try to change it using hook_form_FORM_ID_alter().

// Replace FORM_ID with commerce checkout form id
function hook_form_FORM_ID_alter($form, &amp;$form_state, $form_id) {
  $form['fieldset_item']['mymarkup'] = array(
    '#type' =&gt; 'item',
    '#markup' =&gt; 'My Markup Here.',
  );
}