If you went to coles and they over-charged you by $18 on something you wouldn't be a little ticked off and expect it back?  Ok it's not exactly the same a real B&M store but if you want to have a good online reputation then I'd be apologizing and offering the refund.  The red dot your buyer is likely to give you and the low star rating you are almost guaranteed to get will follow you around for a long time.