public function addCustomItem($name, $sku, $description, $price, $qty = 1)
    {
        $body = [
            'data' => [
                'type' => 'custom_item',
                'name' => $name,
                'sku' => $sku,
                'description' => $description,
                'quantity' => $qty,
                'price' => [
                    'amount' => $price
                ]
            ]
        ];
        return $this->call('POST', $body, $this->getReference() . '/items');
    }