gistfile1.php

function billysbilling_create_invoice_confirm($form, &$form_state, $order) {
  $form['order_id'] = array('#type' => 'value', '#value' => $order->order_id);
  return confirm_form($form, 
    t('Are you sure you want to invoice order @id?', array('@id' => $order->order_id)),
    'admin/commercer/orders/' . $order->order_id . '/billysbilling',
    t('This action cannot be undone.'),
    t('Invoice'),
    t('Cancel')
  );
}

function billysbilling_create_invoice_confirm_submit($form, &$form_state) {
  if ($form_state['values']['confirm']) {
    $order = commerce_order_load($form_state['values']['order_id']);
    $order_wrapper = entity_metadata_wrapper('commerce_order', $order);

    if ($billysbilling_invoice_id = billysbilling_create_invoice($order)) {
      $message = t('Successfully invoiced order @order_id as invoice id @invoice_id', array('@order_id' => $order->order_id, '@invoice_id' => $order_wrapper->billysbilling_invoice_id->value()));
    }
    else {
      $message = t('An error occured while trying to invoice order @order_id as invoice id @invoice_id', array('@order_id' => $order->order_id, '@invoice_id' => $order_wrapper->billysbilling_invoice_id->value()));
    }
    watchdog('billysbilling', $message);
    drupal_set_message($message);
  )

  $form_state['redirect'] = '<front>';
}

/**
 * Page call back
 */
function billysbilling_pagecallback($order) {
  $order_wrapper = entity_metadata_wrapper('commerce_order', $order);

  // Order state should be higher at least checkout complete.
  $not_invoicable = array('cart', 'checkout');
  if (in_array($order_wrapper->state->value(), $not_invoicable)) {
    return t('Order @order_id has order status "@status" and could not be invoiced.', array('@order_id' => $order->order_id, '@status' => $order_wrapper->state->value()));
  }

  if (!$order_wrapper->billysbilling_invoice_id->value()) {
    return drupal_get_form('billysbilling_create_invoice_confirm');
    /*
    if ($billysbilling_invoice_id = billysbilling_create_invoice($order)) {
      return t('Successfully invoiced order @order_id as invoice id @invoice_id', array('@order_id' => $order->order_id, '@invoice_id' => $order_wrapper->billysbilling_invoice_id->value()));
    }
    else {
      return t('An error occured while trying to invoice order @order_id as invoice id @invoice_id', array('@order_id' => $order->order_id, '@invoice_id' => $order_wrapper->billysbilling_invoice_id->value()));
    }
    */
  }
  return t('Order @order_id has been invoiced as @invoice_id', array('@order_id' => $order->order_id, '@invoice_id' => $order_wrapper->billysbilling_invoice_id->value()));
}