Subform action to external url not working

I tried to make a paypal form as a subform:

$form['paypal'] = array(
  '#type' => 'subform',
  '#subform_id' => 'subform_paypal',
);

And the subform callback:

function subform_paypal($form, &$form_state) {
  $form['#action'] = url('https://www.sandbox.paypal.com/cgi-bin/webscr');

  $form['business'] = array(
    '#type' => 'hidden',
    '#value' => 'myemail@mycompany.com',
  );

  ...

  $form['actions'] = array('#type' => 'actions');
  $form['actions']['submit'] = array(
    '#type' => 'submit',
    '#value' => 'Buy Now',
  );

  return $form;
}

When I click the 'Buy Now' button, the subform does not process to external url. Can someone please help? Thank you.