Rob2025
Credit card issues with online shops are often related with a not enabled 3D Secure 2 (3DS2) security feature.
This won’t happen on-site, as 3D Secure is not used with EFTPOS terminals. That’s because you are physically present with the card, so your identity as rightful card owner could be verified in a different way.
I’d recommend to visit the SBB travel centre at the airport right after landing, where you would clarify / sort out the issues you already had when attempting to get the half fare card online.