I would say its fair if you get the 100 price and for him to eat it up. A proper transaction would have involved Coz clarifying the price you emailed, then paying up.
This is ONLY part of the good customer service everyone talks about, myself experiencing it as well. I cannot believe all the posts asking you to eat the cost, when clearly it was Coz who should have seen this error as you were the last one to communicate. Mistakes happen.
I wonder about the IQ of the people posting here sometimes. This herd/bandwagon nonsense gets annoying.