My thought is that a buyer purchases an item based on the total price as per the listing inclusive of postage. If the seller has understimated the postage, and ends up out of pocket, then the seller needs to wear that cost.

If it were me I would refund the buyer the total they paid for the item ie. $57 as well as their cost to return it (providing it is actually faulty) upon receipt. I would have suggested that you advise the buyer if the item is found not to be faulty upon receipt then you would refund the purchase price only of $42. It is best to wait for the item to be returned prior to issuing a refund.

Had the item been listed with the correct postage then the buyer may have elected not to purchase it at the higher price. You cannot expect a buyer to pay for your error with the postage calculation.

Is the fault a clearly visible one? If so, there is also the option to request photos of the damage rather than requiring an item be returned if it is in a state that it cannot be salvaged. That saves the buyer the inconvenience of returning the item, and you the expense of paying for return postage.