Update CSV from WooCommerce shop orders in functions.php file

I’m trying to create a function that will keep a record of WooCommerce orders for a particular product (in this case a magazine subscription)by appending the details for each order with this product in a CSV file stored on the web server. I cannot see any errors in the code but it is not working and cannot understand why.

add_action('init', 'subs_csv');
function subs_csv( $order_id ) {
    if ( ! $order_id ) {
        return;
    }
    global $woocommerce;
    $order = new WC_Order( $order_id );
    $items = $order->get_items();
    foreach ( $items as $item ) { 
        $product_id = $item['product_id'];
        if ( $product_id == '18801' ){
            $subscriber = 'yes';
        } else { 
            $subscriber = 'no';
        }
    }
    if ( $subscriber == 'yes' ){ 
        // Fields for CSV
        $order_number = $order_id;
        $order_total = '￡' . $order->get_total();
        $order_time_date = date("j F Y, G:i:s A");
        // Shipping Details
        $first_name = $order->shipping_first_name;
        $last_name = $order->shipping_last_name;
        $addr_company = $order->shipping_city;
        $addr_1 = $order->shipping_address_1;
        $addr_2 = $order->shipping_address_2;
        $addr_city = $order->shipping_city;
        $addr_state = $order->shipping_state;
        $addr_postcode = $order->shipping_postcode;
        $addr_country = $order->shipping_country;
        // First / Last Issue
        $first_issue = 0;
        $current_mag_args = get_posts(array('posts_per_page' => 1, 'post_type' => 'product'));
        foreach ( $current_mag_args as $current_mag ): 
            $product_id = $current_mag->id;
            $issue_number = get_field("issue_number", $product_id);
            $first_issue += $issue_number;
        endforeach;
        $last_issue = $first_issue + 3;
        // Timestamp
        $order_timestamp = time();
        // Open CSV for writing
        $line = array($order_number, $order_total, $order_time_date, $first_name, $last_name, $addr_company, $addr_1, $addr_2, $addr_city, $addr_state, $addr_postcode, $addr_country, $first_issue, $last_issue, $order_timestamp);
        $file_path = site_url() . 'famtrav-subs-export.csv';
        $subs_csv = fopen($file_path, "a");
        fputcsv($subs_csv, $line);
        fclose($subs_csv);
    } else { 
        // do nothing 
    }
}

Any help would be much appreciated.

Many thanks in advance.