In my experience, I've found no difference in the ride quality between carbon and aluminum seatposts.
Moreover, even if there is any theoretical dampening benefit to a CF seatpost, the less the post is extended, the less of an effect. Thus, with only 3" of post extending, I think it is very unlikely that she would notice any difference in ride quality between cf and al.
5lbs less tire pressure would have a dramatically more noticeable effect on ride comfort.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.