public function process_order( $order_id ){

        // don’t process the ads if they were already created
        /*if( get_post_meta( $order_id, 'advanced_ads_selling_processed_order', true )){
            return;
        }*/

        $order = new WC_Order( $order_id );
        $items = $order->get_items();

        $_item_key = 1;
        $has_ad_item = false; // flag if at least one item is an ad product
        foreach ($items as $_item_id => $_item) {
            do_action( 'woocommerce/cart_loop/start', $_item );
            $product = wc_get_product( $_item['product_id'] );
            if ( $product->is_type( 'advanced_ad' ) ) {
                // create import xml
                $xml = $this->create_import_xml( $_item, $order, $_item_id, $_item_key );
                // create the ad
                Advanced_Ads_Import::get_instance()->import( $xml );
                $has_ad_item = true;
            }
            $_item_key++;
            do_action( 'woocommerce/cart_loop/end', $_item );
        }

        // save hash for ad setup page
        if( $has_ad_item) {
            update_post_meta( $order_id, "advanced_ads_selling_setup_hash", wp_generate_password( 48, false ) ); // Ad Setup Hash
        }

        // notify client after the purchase
        Advanced_Ads_Selling_Notifications::notify_client_after_purchase( $order_id );

        // mark this order as processed with a custom value
        // update_post_meta( $order_id, 'advanced_ads_selling_processed_order', 1 );

    }