So I goofed up. I booked a ticket for next month on the SBB site. I did not notice that the 1/2 fare box was checked, so I only paid half price. However I do not have a half-fare discount card, and purchasing one would not be cost effective.
What are my options? Should I just board the train as normal, and then explain to the ticket-agent on board that it was a mistake? Will they let me pay for the difference on-board?
The online ticket can not be exchanged or refunded, so I can't cancel and start over.