MC4WP_API_v3::update_ecommerce_store_cart PHP Method

    public function update_ecommerce_store_cart($store_id, $cart_id, array $args)
    {
        $resource = sprintf('/ecommerce/stores/%s/carts/%s', $store_id, $cart_id);
        return $this->client->patch($resource, $args);
    }