When you first launch Cards, you see the familiar iOS notification that the app would like to send you push alerts. But why? What could Cards possibly need to notify you about? Sadly, it’s not a reminder about your Aunt Matilda’s upcoming birthday. Instead, Cards can send you a push alert when your mailed greeting is on the U.S. Postal Service mail truck for delivery.

That’s because Apple prints a bar code on the envelope before it sends your card—specifically, a USPS . When your envelope is set to be delivered, the post office scans the code, and your push alert is triggered.

The Cards app includes one travel theme that inserts a virtual ticket atop your photograph. When you use that theme, the app automatically detects your current location and digitally inserts it on the ticket. Even better, when you add your photo, Cards looks for any GPS data associated with the image, and updates the location accordingly. That way, if you only remember to send a card from your European vacation when you’re back in the States, the shot of you holding up the Leaning Tower of Pisa won’t claim you snapped the photo in New Jersey.