Am I correct that you are creating orders directly in Kentico admin area and this info is used for calculation then?
I think you can try this one:
var cart = ShoppingCartInfoProvider.GetShoppingCartInfoFromOrder(calculationData.Request.OrderId);
But I'm not sure if Kentico creates a shopping cart for directly placed orders in admin, have never tested it myself. If the line of code returns
null for me that would be an indicator that Kentico actually doesn't create shopping card for manually created orders, but you can check in the database to be sure.