Just want to check really. I ordered 3 items together on BIN. The next day I had a message to say they might have only sent me 1 x item instead of 3 x items I ordered, they got in a tizz at Post Office. To let them know.
So parcel arrived - completely different thing has been sent, obviously someone else's parcel 
I am not paying to return a random wrong item. I have sent a nice message asking them if they want to send me a prepaid envelope to return the wrong thing in.
I haven't opened a case, it's clearly an error. But they can't make me pay (its around a 450g parcel) to return an item sent in error can they?