All I can suggest is that you mail this person again saying you meant to mail it to someone else. There are people out there that will be kind enough to send it back, not everyone will however. You can even tell them they can feel free to keep 10% finders fee or however you want to call it, that way if they agree you at least have some back.
But it's true customer support isn't allowed to help you with that, they may not even be able to help. It's a matter of design choices and resource allocation. It's easy to say "Hop on quick and undo my mistake", but there are many that send in tickets for that, they help one they have to help all. And before they can undo it they have to verify it, and that can often be impossible as they don't know what players agreed on.
They spend one or two minutes helping you, then an hour to verify the other one's claim that it wasn't your mistake but part of a deal, then they have millions of similar issues to deal with. It's the smart move for them not to involve themselves in player mistakes. It does however mean you have to be careful where you mail things to.
So my suggestion is to try contacting the player you send it to, either that or accept that you may not see the credits again and start gathering new ones.
