xiaoz, below is the reason tranny service is not in the manual. This comes from the workshop manual Isuzu Tech's use.
I would ask for an eval of the fluid first, or take it to a good tranny shop if you don't trust that dealer. I've had my tranny serviced twice by two Isuzu dealers. The first time I had the filter/screen/strainer replaced and it brought the total up to $256. The next time I just got a tranny flush 'n fill for $115. The 'filter' is more like a screen and probably doesn't need replacing very often. That part costs $73.
I've had the front bearings repacked twice for around $200.
I hope this helps you out.
Mark Griffin