https://www.techradar.com/news/facebook-is-fighting-electoral-meddling-with-paper-postcards

As part of its ongoing effort to clean up its reputation, Facebook will soon start using old-school paper postcards to verify the location of anyone buying ads related to US elections.

The postcards will include a unique code, which advertisers will need in order to prove they are in the country. The codes will be necessary for anyone who wants to place an ad mentioning a candidate running for Congress.