The following snippet is from https://help.twitch.tv/customer/portal/articles/2927987-earning-revenue-from-in-extension-bits-beta:
Q. Can my viewers get returns for Bits used in Extensions?
A. Per our policy, Twitch does not offer returns of Bits used in Extensions. Exceptions are handled on a case by case basis - please contact Customer Support. Partners and Affiliates will have the same protections from Bits chargebacks as they do with Cheering in Chat.