How do you know?
Because the book states no one was paid. The letter makes it clear that they do not want anything in return. After having read the book I believe the author of the anonymous letter wasn't paid.
If you have anything that goes against that - I'd be interested in seeing/hearing it but I don't think you do. I think your dislike of PH is clouding your view on what is a pretty clear matter. I don't particularity like PH either but it seems clear to me there was no need or reason to pay an anonymous letter writer.