public function add($id, $name, $price, $quantity, $options = [])
    {
        $cartItem = new CartItem($id, $name, $price, $quantity, $options);
        $uniqueId = $cartItem->getUniqueId();

        if ($this->content->has($uniqueId)) {
            $cartItem->quantity += $this->content->get($uniqueId)->quantity;
        }

        $this->content->put($uniqueId, $cartItem);

        return $cartItem;
    }