Depending on how it goes with Costco, you might want to call
Disneyland ticketing. Not the same situation, but I bought an inactive EGift certificate from Costco last year. When I called Costco, they said they couldn't give me a refund. I took it up the chain and it was never going to be refunded by Costco. I ended up calling Disney gift card services and they were able to fix it for me. However, I had an actual gift card # and pin that they were able to look up. I realize that you don't have any numbers that can be looked up.