I am pretty surprised to hear that I wouldn't be guaranteed my seats for World Series. Because I'd better get my seats for World Series. Will be super-super-super upset if Nats made WS and I was forced to 300 level. FYI my prediction would be about double the price of whatever a non-STH premium game price is for your seats. So for the $60 dugout seats that's $200. Sounds about right. I remember getting some basic info about playoffs in 2005 but not an invoice -- nothing like an invoice at all. Could be wrong.
Edit: looked around on other websites and I feel confident that the vast majority of full-season STHs will be able to purchase their seat locations for the World Series. Reds and Giants guarantee it 100%. Cardinals almost guarantee it. Yankees basically say we'll do whatever we can that's reasonable to get you your seats but MLB controls. Those are the only 3 I looked at. BTW our STH benefits suck -- except of course for Red Carpet Rewards. But if that gets removed or reduced, I want a whole slew of stuff like access to the field for batting practice, private autograph sessions, etc. like other teams.