I think it would depend on two things. First, do the coop's governing documents give the board of directors the power to impose such fines? Second, has the board (or the managing agent on behalf of the board) sent a notice to shareholders alerting them that as of X date fines will be imposed on late payment of nonmaintenance fees? In our coop, the governing documents give the Board the right to impose fines on late payment of nonmaintenance fees but it has chosen not to impose them. We've never had an instance when someone has paid their maintenance but refuses to pay our trivial monthly storage cage licensing fee. If your building's fees are more substantial, I could see out of fairness to the majority of shareholders who likely pay what's due on time that those who pay late get fined.