code: public static function setGateway($gatewayKey)
      {
          if (!array_key_exists($gatewayKey, Config::get('shop.gateways')))
              throw new ShopException('Invalid gateway.');
          static::$gatewayKey = $gatewayKey;
          static::$gateway    = static::instanceGateway();
          Session::push('shop.gateway', $gatewayKey);
      }
docs: Checkout current user's cart.