I don't disagree with using postcards, but maybe you should rectify your use in them. A postcard is something that people normally send on holiday, and companies send them during the holiday season to their existing customers, like a "hello, we still exist", meaning.
If you wish to send postcards maybe you should send them to existing clients during the holiday season, so Christmas and Easter. I would not send a postcard to somebody you don't know, as it's likely to be ignored. This would be advertising to existing clients.
Hope this helps.