Hello MS320
thank you for your question.
Usually the SBB Contact Center takes payments by phone with credit cards and they send the ticket per post, or they take the credid card information as guarantee for the reservation and you pick up the tickets at any train station in switwerland.
Since you are in Slovenia, I suggest the second option.
1. call the SBB Contact Center and make the reservation
2. give your credit card as a guarantee: you will receive an order number
3. pick up the tickets once you arrive in Switzerland by giving the order number to the colleagues at the station
4. pay with whichever method you prefer.
Regards
EleonoraS