Weird for the USPS to do it, but hey I guess someone has to.
For what it's worth if some form of postal or online voting is used, it absolutely has to be cryptographic in nature.
Everyone has to be able to verify that the results are legit (match the result against a public key of each poll). You have to be able to verify that your vote was counted, either at the time of casting it, or after the fact.
How you verify that only those eligible to vote can do it, that's the tricky question that no technology except maybe a tattooed QR code, or NFC implant can solve.