It looks like you've sent your last 100-percent cotton letterpress card. Apple on Tuesday quietly discontinued its iOS-based service for sending photo postcards via good ol' fashioned snail mail, removing the related iOS app from the App Store.
Should you still have the app installed on your phone, you'll receive the following message when you attempt to create a new card:
The Cards service is no longer available. You can review your previous purchases in Saved Cards.
Born in 2011, Cards was a relative of the late iCards, which passed away in 2008, and is survived by iPhoto's card and calendar printing service. Pour one out in memory of the dearly departed app and service--it was taken from us too soon. We can only hope it joins iCards in the hallowed Hallmark halls of heaven.
In lieu of flowers, please send condolence cards to 1 Infinite Loop, Cupertino, CA, 95014.