Apple has decided to discontinue their Cards iOS app service. September 10, 2013, was the last day it was usable. The service is no longer available.
If you were someone who used, and enjoyed, the Cards app this news may come as a disappointment. Apple says that you will still be able to view your previous purchases by opening the app and tapping “Saved Cards”. That is the only function that the app will continue to have.
What are your options for replacing the app? Apple suggests that you use iPhoto instead. In the announcement about the discontinuation of the Cards iOS app, Apple points out that you can order letterpress cards, customized with your photos and text, through iPhoto.
You place your order, and the cards will be shipped to you “through a carrier such as FedEx rather than through the United States Post Office”. I’m not sure why Apple has decided against using USPS, but there you are. Apple has a page with lots of information about iPhoto Print products that you can check out if you are considering using iPhoto as a replacement of the Cards app.