WooCommerce virtual orders can be automatically marked as ‘completed’ after payment with a little bit of code added to a custom plugin, or your themes functions.php file. By default WooCommerce will mark virtual-downloadable orders as ‘completed’ after successful payment, which makes sense, but some store owners will want to be able to automatically mark even a virtual order as complete upon payment, for instance in the case of a site which takes donations where no further action is required. To do so, use the following code, which is based on the core virtual-downloadable completed order status:

add_filter( 'woocommerce_payment_complete_order_status', 'virtual_order_payment_complete_order_status', 10, 2 );

function virtual_order_payment_complete_order_status( $order_status, $order_id ) {
  $order = new WC_Order( $order_id );

  if ( 'processing' == $order_status &&
       ( 'on-hold' == $order->status || 'pending' == $order->status || 'failed' == $order->status ) ) {

    $virtual_order = null;

    if ( count( $order->get_items() ) > 0 ) {

      foreach( $order->get_items() as $item ) {

        if ( 'line_item' == $item['type'] ) {

          $_product = $order->get_product_from_item( $item );

          if ( ! $_product->is_virtual() ) {
            // once we've found one non-virtual product we know we're done, break out of the loop
            $virtual_order = false;
            break;
          } else {
            $virtual_order = true;
          }
        }
      }
    }

    // virtual order, mark as completed
    if ( $virtual_order ) {
      return 'completed';
    }
  }

  // non-virtual order, return original status
  return $order_status;
}

Note: This article has been updated to work with WooCommerce 2.0+ and the sample code above will no longer work with WC 1.6.6 and previous

Remember, this functionality is now available as a convenient plugin with the release of the premium WooCommerce Order Status Control