MC4WP_API_v3::add_ecommerce_store_product_variant PHP Method

    public function add_ecommerce_store_product_variant($store_id, $product_id, array $args)
    {
        $resource = sprintf('/ecommerce/stores/%s/products/%s/variants/%s', $store_id, $product_id, $args['id']);
        return $this->client->put($resource, $args);
    }