imsayin said:
I disagree with this. First of all, he lost his listing fees. Secondly, at this time of year, you can get a higher price for your merchandise, so if he can't relist the item in a timely manner before the holidays, he may lose money in product value. JMO.
He can file a nonpaying bidder complaint and get his fees back - which, is what I stated in the response your are quoting. I assumed (yep, I know what assuming means

that the buyer contacted the seller immediately - like, as soon as her son purchased the sleeping bag. It would only take the seller a matter of minutes to relist and it also sounds like (based on OP's statement) that he responded fairly quickly to her response.
Anyway - I still think the guy is only out his time...IMO. He may have lost a couple of hours as far as his sleeping bag being available for purchase.
I personally have about 600 feedback comments on my Ebay account. I'm a fairly experienced ebayer - been shopping there since 1998 - back when a majority of the people were fun to deal with. Sadly, I've experienced more bad than good over the past couple of years.
When I shop on Ebay - I'm typically on a shopping binge. I buy 3 or 4 things withing a matter of minutes. If I plan on trying the last second sniping game -I log on Ebay and from that moment on, I do not have to enter my password again because I've clicked the "remember my password" box.
I do believe this incident was a mistake. Heck, I've watched adults click through stuff on accident. Some people are just fast computer clickers. They see the buttons and click away. It would be extremely easy for OP's son to click his way through a purchase purely accidental.
I also really feel bad for the Ebay sellers that have to deal with all of the excuses they more than likely get - but, geez - OP was looking for advice - not flames and accusations
OP: You need to do what you feel is right for you. The seller will get his final "win" fees returned & he can relist. Even if he paid additional money for services that he will not get reimbursed - well, he would be out that money if the sleeping bag doesn't sell anyway. He knows this when he listed the item for sale. I do feel $15 is too much since this item was only $35. If you truly believe you owe this guy anything - I'd go with $5-6 as another poster recommended.