One issue is who pays the processing fees for the deposit & refund transactions. HackerOne could work around that issue by copying the practices of video game "microtransaction" payments: sell "report points packs", say 2500 points for $25 minimum in a pack. User needs to deposit 100 points to report, for each report they open. If the report is accepted they get their 100 points back, if not they lose their 100 points. If they want to open more than 25 reports at once they need more points packs. The $25 pack is non-refundable, so there's no added transaction fee for the refund.