Personally I only charge actual POSTAGE costs.
If I've charged a buyer £10 for postage and it works out at £9.85 I don't bother to refund them, but normally I just assume the time it takes me to get to the PO etc is just part of the cost, I don't specifically add as an extra.
If it was me, I'd be saying to him postage cost "11.95" I paid you "24.95" I'll be having 13 bucks back.
IF the package was well wrapped etc and he was a decent seller I'd be tempted to be flexible.
If it's Ebay just forward his message/email on to them and claim the 13 bucks back.