What matters is where that mobile device is and if that device is in a place where you would want to reach individuals in that place with an ad impression.
From copy and design to print, a postcard can get in mailboxes quickly without additional steps required for more complex self-mailers or envelope packages.
Many marketers, who have been around a minute, lost the love for QR codes 5-10 years ago, but the use of QR codes has skyrocketed in Millennial and Gen Z pop culture.