The USPS is unlikely because it would be too difficult to be kept secret considering the left-leaning employees there.  Also, the ballots are sealed in envelopes that reveal the name and address of the voter, so any attachment of unique tags would violate the secrecy of the vote.  The post office never actually sees a ballot...only envelopes.  The ballot is sealed within an inner envelope to preserve the anonymity of the vote. Wanna get crazy?  The inner envelope had a self-stick seal, but you licked the outer envelope with your name and address on it.  Could they be collecting DNA samples that way?  I used a wet paper towel in a moment of paranoia.  LQL!