We need ballots delivered, but I don't see why the USPS is the only option for delivering them. Why does it have to be the USPS? There are plenty of other companies that are very good at delivering mail.
Besides, the law is no guarantee. The USPS loses plenty of packages. You have no real recourse. What are you going to do, take them to small claims court? Make the government count your ballot anyway (assuming they believe you)? Good luck with that. Such guarantees mean nothing the real world.