Exactly this. I got a call saying the balance was now overdue and they were going to cancel the ticket if I didn’t pay within 24hours. I explained that it was highly unlikely we were now going to take the trip. They said there was no need to call back as it would automatically get cancelled if I didn’t pay in the timeframe. When I’ve done this before that’s early what happened and all trace of the trip disappears within a couple of days.