I wouldn't issue any refund.

I would explain that it was an ebay calculated cost, that it was in the listing and he bought it under those conditions. And I would add that threatening me with bad feedback if I did not make a refund that was never menioned in the ad amounted to feedback extortion, which was against ebay policy and they could be reported for that.

I agree with what lezned said

Threatening negative feedback unless you comply with their request is breaching the "Feedback policy". Any negative feedback they leave will be removed upon request.

I would suggest reporting the buyer immediately. Their behaviour is totally unacceptable.

Go ahead now, report them.

If they do go ahead and give a neg, get onto ebay help line chat and ask to have it repmoved under their policy of no extortion and offer to show them the exchange. Explain it is a neg because they bought and now demand a refund on the previously listed postage.

You'd have a good chance of having it removed but even if you didn't, it won't kill you to have 1 neg and you can always reply to it.