If YOU made an error in the posting, then I would refund the original shipping. HOWEVER, if the buyer now has remorse for whatever reason, the buyer pays return shipping and only is refunded the item's final selling price not including the original shipping (unless the listing said free shipping).
If this was a local sale and the buyer drove to your business to pick it up, would you pay for the cost of the buyer's gas? No. If the buyer brought it back and you refunded the purchase price, would you again pay for the buyer's gas? No. The transportation cost is borne by the purchaser unless the seller offers to include shipping in the purchase price.
My opinion is you do not owe the remorseful buyer a refund of any shipping charges either way, only the final selling price as per your 3 day refund policy. The buyer had all the time in the world to research the item you sold to see if they could buy a new one for the same price. The buyer did not do their homework first and so now they must pay a little for their remorse. JMO.
I looked at your listing and some other MARS paperweights and I see that each one is unique just due to how thay are made. One is a little cheaper than yours but I do not know if it looks as good and it has no box. What is this buyer going to save for all their hassle, $5-10? I also see some selling for more than yours and this buyer bid 4 times on your paperweight to get to the final price. It was not like it was a spur of the moment bid.