It probably would have been a hassle for them to put the drink through the till anyway, since they didn't know what he had. Maybe it's the type of shop where it's just a flat charge for any drink and they don't differentiate them on the tills, but where I work you'd have to put the exact order through to keep the inventory correct, and it would be a pain in the ass if you didn't know what he was drinking.
You probably saved the till person some bother, and saved yourself a bit of time since it definitely would have caused a delay and been a whole thing.