I'm a giant Piper fan but I don't ever want to see him at Joe con only because last time he was at a con in Toronto he was literally charging for everything you could imagine. And I mean everything!!!
Then that's the arrangement he (or his booking manager) made with the organizers of the show/con.
Notice, Zack Hoffman didn't charge a fee, neither did Larry, Mark, or anyone else.
When I contacted Slaughters agent a couple of years ago. I was given two prices. One for him to sign for free, and another lower price if he charged for his autographs himself.
Pretty standard, really.
I met Piper in 2005. No fees for anything.