That's fine.
But, if dude would have said - I agreed to this transaction under the terms that were in place when I agreed, and those terms were free shipping - he would have been right and you would have been wrong.
It's supposed to be on ME, the buyer, because you didn't sufficiently proofread your listing before publishing it? How's that sound?...
There are two issues. One, is it an honest mistake. Two, is it your fault?
In your case, I'm sure it was an honest mistake. Doesn't mean you get a do-over though. Dude was nice enough to give you one; that's great.
In this guy's case, we're saying that circumstantial evidence strongly points toward it not being an honest mistake at all.
Sorry for carrying this on - I'll chill now. ...Just can't stand people who renege on transactions, from either side.