Mobipium "Tax Proof" for Wire Transfers

[Ack]

I'm trying to get paid by wire transfer from Mobipium, but their finance dept is saying "since you're in the US, we need a Tax Proof to pay you by wire".

I have no idea what that would be. I asked what specific document they need, but haven't got a response yet.

So, for anyone living in the US and getting paid by wire transfer from Mobipium, do you know what they are asking for?

@Luke Did you have to do this?