I used to run a SaaS, and I also used postcards to try to promote it! Why not use emails? I was sure that emails would get spam-collected, but physical postcards might get some attention.

I don't know if the tactic worked.

These days, if I were mailing postcards, I'd make sure to add a special QR code to them. That way, if someone went to my sales page using the QR code, I'd have an idea that the postcard had been seen by the right person. Postcards are rather expensive (both the postcard and the stamp). Who wants to keep trying that without knowing it was successful?