As a buyer, I would never expect postage refunded back to you, the same as if I bought something from Harvey Norman I would not expect to be reimbursed for petrol to take it back.

The cost of post back to your client is your cost I would imagine and the misteach in the refund is also a cost you would be better off wearing as it leaves a bad taste to keep going back and saying "sorry another misteach by me":.

The whole idea is to get a repeat customer and dazzle them with your service, professionalism and honesty.