Need some eBay advice. Seller sells working item, and buyer says it doesn't work.

Here's the deal.

I sold an item (Star Wars Clone Wars) for the LeapPad Explorer. This item works fine in my Son's Leapster Leappad without an issue. It sold and I shipped right on time. The buy states it does not work in their children's Leadster Explorer (Note that this games box states for Leapster Explorer Learning game). They state it should work in both the Leapster Explorer AND the tablet which is all I know it works in. When they place it in their children's Leapster Explorer (Non Tablet) it gives a blank screen. I went through every necessary thing I could and I asked them to ship it back to me and I would cover the shipping if it doesn't work. Well I receive it today and it works fine in the Leappad Explorer. What should I do in this case? I don't want negative feedback, since I have a perfect record on eBay. To be fair, since it does work, I will accept the return of the items price minus any shipping to/from since it works. What do you guys think? Basically sold an item that works, customer receives it, and uses it in a different device other than mine, and would like a refund. I can't find online anywhere people discussing this. They state the game is supposed to work in both. I'm clueless..

Thanks in advance for any advice to get this settled!

You refund, thank them for their business, apologize for the inconvenience, and wish them well. Both sides tried to do the right thing. If you want to go above and beyond, cover the shipping, and they'll love you forever. It's not worth the time to chase it further.

I explained to them that I would refund the item amount but since it wasn't not defective, I will not refund the shipping. All in all trying to be fair. I even sent them pictures of it working. Just waiting for a response. Thanks for the advice!

UPDATE: After viewing the pictures I showed the customer of the item working in my system, we both came to terms that they will not get the shipping refunded. I went ahead and cancelled the transaction after that for fee refunds, etc. and all is good... Thanks for all the feedback!