But why would Domino have to sell
the nun a Double-N ticket? Surely she would have had one wating for her? Unless, of course, she was a very bad nun...
The way I always saw it is that the better life you lead, the better your travel package. But if you haven't led a good life, you can upgrade your package, as Manny offers to Bruno. I thought that this would be the 'legal' way of getting a Double-N ticket. Nick and the others had all bought tickets meant for other souls, which is why they never made it to the Ninth Underworld.
That's how I see it, anyway. I'm probably wrong, though, I usually am