That is one funky looking seat post. It is designed "to change a 73 degree seat tube into a 78 degree seat tube". Clearly for someone who does triathlons and wants a more forward position that is generally used with areo bars. This seat post would not be comfortable for normal riding and would most probably put your knees waaay in front of the pedal axle.

IMO, $300-350 would be reasonable and ask for the old seat post and maybe the original saddle as well.

Also, your daughter's reluctance to shift sounds like confusion, fear of shifting or not wanting to be told by her parents how to ride. Perhaps you should have a heart to heart with her and offer to take her to a big parking lot where just the two of you can practice. My wife wouldn't shift for almost two years just because it was confusing to her and frankly, the tone of my voice didn't help matters.

Good luck!