I agree with you OP, if a seller has admitted that they have sent the wrong item then you should not be out of pocket for expenses for return.  IMO they should let you keep the goods and send your correct purchase or reimburse you for post costs to return or arrange collection of incorrect goods and drop off of goods you purchased. 

Not sure about the comparisons to petrol expenses etc, I think that buying from bricks and mortar shops is very, very different from buying on-line, you do not have the chance to inspect your bag to make sure correct goods are in there - on-line you are entirely reliant on the sellers actions to send the correct goods.