mark_charge_success is called from record_charge. The exchange table is updated by `amout` where paydays table is updated by `charge_amout`. These two values are sent to record_charge as params. The relation between the two is [`amount = charge_amount - fee`](https://github.com/gittip/www.gittip.com/blob/95a9aa43164a3410eece747efbafe52569b5040a/gittip/billing/payday.py#L561). I must say I don't really understand why we have this but I guess there was a reason (rounding?). Anyway, the code above is correct.