I agree that you are morally right, but I am not certain of the precise legal position.
I feel that if they send you what you ask for and you return it, that is not their fault so you should pay the return postage (bearing in mind you generally have the right to return goods under the Distance Selling Regulations), but if they send the wrong thing you, similarly, should not be out of pocket.
In your case, they are unable to fulfil your order, so you should have a refun of what you originally paid (including postage) and should be reimbursed for any reasonable expense in putting right their mistake (or at least the postage cost).