While I don’t particularly like being upsold on a rental car when booking a flight (or an extra-leg-room-seat, or other bullshit perks that make buying a plane ticket online take five times as long as it should), I do kind of appreciate the blatant way Spirit goes about doing it.
The next page, incidentally, was http://www.spirit.com/make-em-pay-for-a-ticket-then-again-for-a-seat-assignment-then-again-for-even-thinking-about-bringing-a-carry-on-and once-more-to-ensure-the-plane-both-takes-off-and-lands. After that, it was just http://www.spirit.com/sucks.
[Thanks, Slo]