ThankYouPro from Zendoa LLC is a new app, available from the iTunes app store. The app is free but there is a charge of US$2.99 for each card they mail for you.
ThankYouPro icon
It is found in the Lifestyle category and was updated June 4, 2012. It is available in English only and requires iOS 4.2 or greater. It works with iPhone, iPod touch, and iPad.
The app offers over 20 card designs from casual to formal. You also can create your own from a photograph or image.