To catch the attention of potential clients, the method of self-promotion has found in the postcards a very valuable tool to be used by those who want to be noticed. However the collectible value of the postcard itself is what makes them risky sometimes. you can't choose the person who will be interested in the card or even know if it will reach your 'target' but...once you get a single person to notice your work and take it home, then the hardest part of the job is essentially done.
