How did they pay - only option I see is a possible chargeback
Is that not unethical since the business hasn't actually done anything wrong and the item isn't faulty?
Goods are "faulty" as the wrong size has been supplied i.e. goods not as ordered so I see nothing unethical in requesting a chargeback if they wont respond.
Ahh right I misread the size issue (thought the customer had ordered the wrong size themselves rather than sent the wrong size) my mistake. Then yes a chargeback and making the dress available for collection by the business wouldn't be unethical I agree.
Just a point that's nothing to do with the OP but is part of the discussion I would say that is frequently confused with mail order. If you make a chargeback or have another item sent to you (which can happen for a variety of reasons, not least slow post). The unwanted item is not the buyers property and must be returned or at least made available for collection. It's worth putting that in terms and conditions that you retain the rights to faulty and cancelled goods as well as quite a few people think they can keep the extra one.