It's free to download the app, and many of the designs are free. Depending on how many postcards you send, they cost anywhere from $1.99 (for one) to 49 cents (for 1,000 or more). However, you can send the postcard electronically for no charge.
The app can be used for postcards, invitations, greeting cards, notecards and more.
Take a look at what they have going on at Red Stamp.