"Fraud with mail-in ballots would be orders of magnitude easier to pull off."

As recent bombers have discovered, this isn't the 70s Unibomber era anymore, and every piece of mail is individually bar code tracked and enormous computerized fingerprint databases exist along with CCTV footage and cell phone monitoring databases. Using the US mail to commit a crime in a general sense (not strictly election fraud) would have been fairly untraceable in 1970 but half a century later it would be an extremely foolish strategy. The USPS is slightly less locked down than my bank, but much more locked down than my local food store, for example.