Presumably you have a single item number for your "non-stock item". Move it to a different control account and specify "User defined costing". That's how your issue is supposed to be handled.

The more general comment about Editing information on-the-fly has been discussed. However, it's very difficult to tell whether you are making a change just for the single Order, or you want the change reflected back in the master record (whether we're talking about a Customer or and Item).