It seems that the issue exposed by DoorDash and Postmates is easily resolved by inserting volume discount language into the contract with the arbitrator. A competitor to DoorDash's arbitrator has already started making that sales pitch. It's possible the fees could be discounted for these existing disputes, too, though perhaps doing so during the pendency of a dispute might be risky--politically, if not legally. In any event, I wouldn't expect this loophole to stay open for long. Occasions for such schadenfreude aren't usually repeated.