code: public function placeOrder($statusCode = null)
      {
          if (empty($statusCode)) $statusCode = Config::get('shop.order_status_placement');
          // Create order
          $order = call_user_func( Config::get('shop.order') . '::create', [
              'user_id'       => $this->user_id,
              'statusCode'    => $statusCode
          ]);
          // Map cart items into order
          for ($i = count($this->items) - 1; $i >= 0; --$i) {
              // Attach to order
              $this->items[$i]->order_id  = $order->id;
              // Remove from cart
              $this->items[$i]->cart_id   = null;
              // Update
              $this->items[$i]->save();
          }
          $this->resetCalculations();
          return $order;
      }
docs: Transforms cart into an order.
  Returns created order.
  
  @param string $statusCode Order status to create order with.
  
  @return Order