I'm in internet retailing. Have you directly asked for the postage back? You will get the best and quickest response by sending a Recorded letter addressed Managing Director (Private and Confidential). Often it is the sales staff that have either overlooked the refund, don't engage brain sufficiently to realise that you are entitled to it, or have just been slow in actioning the refund. It is unusual not to refund the return postage.
At work it is not unusual for a customer to look at one item and supply the code for another. You haven't given the explicit circs but in those circs I would not refund the return postage either. I'm not saying that's what has happened but it's just an example.