How do I add an additional submit handler in commerce checkout form? How do I add an additional submit handler? I have tried this but it did not work.

function c_coupon_form_alter(&amp;$form, $form_state, $form_id) {
  drupal_set_message($form_id);
  switch($form_id) {
     case 'commerce_checkout_form_review':
      drupal_set_message($form_id . '_form submit');
      $form['#submit'][] = 'c_coupon_review_submit';
  }
}

function c_coupon_review_submit($form, &amp;$form_state) {
  drupal_set_message('submitted');
}
What you are doing is correct.

The problem is that Drupal Commerce has defined the submit action on the buttons that is clicked, which will be used instead of the submit handlers defined on the form itself. So you need to add your submit handlers to the buttons (Cancel, Continue) that you want to use your submit function.

$form['buttons']['continue']['#submit'][]='c_coupon_review_submit';