If you buy a coin from a foreign source and have customs charge....
AND.... the seller offers PAID return shipping..... do they pay the customs fee?
Did you pay a customs charge when you received the coin and are asking if the seller refunds it to you if you return the coin or are you asking if the seller pays the customs charge due when you return the coin to him?
I can't answer for anybody else but IMO, a customs fee is not part of the shipping charge. Shipping is what it costs to get something from here to there and doesn't include government fees.
You could ask them to refund what you were charged for customs, but don’t be surprised if they decline your request. How much was the coin and the custom charges? That might influence the decision.
I believe the seller would refund what you paid on ebay which is not include the customs fee.