Yeah, Stripe only gets so much say over chargebacks with their platform provider and the cardholder's bank, the only platform that I've seen chargebacks successfully fought & won on is Tsys, its always a loser on First Data.

That being said, $15 is a low chargeback fee (damn close to cost the issuing bank & platform charge), generally you'll see a $25 to $30 chargeback fee with a $20 to $75 retrieval fee.