But there is still a fee for ZED? Which usually costs less, ID or ZED?
Yes, there's a fee for ZED, and yes it's only space available. You'll go below regular passriders, passrider family/buddy, etc etc.
It's been a while since I've done either ZED or ID, but ZED was cheaper than an ID. We had ID90 when I used it last.
I was going from LAX-AKL, the ZED fair was like $250 RT, because that's what that mileage zone segment cost. Although depending on the ZED agreement and whether it's a "high" or "low" ZED, etc the price can vary.
The ID was about $700 RT, since it 90% off the highest published "Y" class fare and you're paying 10%.
Both were refundable, so it's a good idea go have both in your pocket. That's under the assumption that there's two carriers serving your dep/dest, and one is a ZED agreement carrier and the other an ID.