Holy cow! What a piece of work! I wouldn't sweat it. Sounds like a blowhard trying to shake you down for $$. You were civil and took responsibility for your mistake, and compensated him for it.
I wonder if this person really is a lawyer like he said. I mean, if he spent a whopping $21 for a gift for one of his "closest colleagues", I doubt that he's all that he claims to be.
Have you checked out the feedback he's left for other sellers? Has he made of habit of leaving negative feedbacks or just seems nasty? Not much you can do now, but at least you can see if he has a history of this behavior. And add him to your Blocked Buyer List asap!
We all get a lemon every once in awhile