It’s a federal crime to tamper with USPS mail. If UPS or FedEx decide not to deliver your parcel you may be entitled to a partial refund.
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.