why not redirect to another page after form submit
If that's a node form, the problem is the node submit handler sets the redirect already in node_form_submit

If you want to redirect to checkout after a node form submit, you can do it by adding a submit handler to the $form['actions']:

/**
 * Implements hook_form_alter
 */
function MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'NODE_TYPE_node_form') {
    $form['actions']['submit']['#submit'][] = 'MODULE_NODE_TYPE_submit';
  }
}

/**
 * NODE_TYPE submit handler
 */
function MODULE_NODE_TYPE_submit($form, &amp;$form_state) {
  // You don't need order_id as /checkout will route to the correct location for user
  $form_state['redirect'] = 'checkout';
}


Note: You could also do it with Rules.