Did you transfer the tickets to the buyer? You are being charged 100% of the sale price which is the penalty charge for not delivering the tickets.
OK so if you mailed physical tickets to the buyer then I hope that you kept proof of that? And not sure if you have to confirm on your sales page that you delivered the tickets, I know with electronic transfers you do.
Anyhow, it does seem that Stubhub charged you for not delivering the tickets. Since it sounds like you did then you need to call them to ask why you were charged a non delivery penalty. Good luck with that, their customer service is awful. 866-Stubhub