src/Extension/OrderManipulationExtension.php
public static function add_session_order(Order $order)
    {
        $history = self::get_session_order_ids();
        if (!is_array($history)) {
            $history = array();
        }
        $history[$order->ID] = $order->ID;
        ShopTools::getSession()->set(self::$sessname, $history);
    }
public static function add_session_order(Order $order)
    {
        $history = self::get_session_order_ids();
        if (!is_array($history)) {
            $history = array();
        }
        $history[$order->ID] = $order->ID;
        ShopTools::getSession()->set(self::$sessname, $history);
    }
Add an order to the session-stored history of orders.