It is odd, I would have thought $0 is the more obvious number there. It seems you can refund via a shell command though, so the money is not trapped

Given they delete the boxes at $0, that may be cutting it close. That said they should add a flag that lets a user run down to the $0 if they want to live dangerously. They could also change it to $1 since that's their minimum balance required for refund.