Thanks for your reply. I am buying the ticket for myself.
I got the Halbtax Plus 2000 last July, and it worked with the app on the phone, but today my phone ran out of battery, and I tried to buy the tickets on the website with my laptop (for this weekend), but I was not able to pay with Halbtax Plus 2000 (my credit remains 1400 CHF).
Please double-check.
BW, LT