IF there was a chargeback, then you would, under the typical commissions arrangement, have to return some of the commission (the share attributable to the chargeback). But you can insist on seeing evidence of it before you pay...and if they won't provide evidence, you can refuse to pay. They could attempt to sue you for the money, but to win their case, they'd have to provide evidence in court: there is no way for them to make you pay without evidence.