You should have your t+c's set out already to comply with dsr, in future simply reply they can return the item if they wish for a full refund within 7-14 days if they are not happy.
You are not giving them anything they are not entitled to do in law, whether you refund the return postage is down to yourself, personally if the item is tat I would, but then again I wouldn't sell tat in the first place.