We believe in open-source technology.
Feel free to browse the code of our website / app libraries to see by yourself what is sent to us.
We never have access to your private key besides its encrypted version at the time we print your card.
Yes, you could use your own card with the app. You will miss some features such as the card name, which is stored on our servers (nothing you can't live with).
If we were to provide the card only, you could argue that 16.99 USD is expensive but we strongly believe that the combo card + app is worth at least the price of two beers (yes, beer is not cheap in Switzerland)