The "Requires two ingots of chest type" thing of the upgrade is not very intuitive.
I see why it's required (so to not be cheaper than the other recipe), but I'd rather just have one material that's used for all sorting chests and the upgrade rather than this.

This 'ask for items' way was quite recently added and I think this is the best way we can do this. This way players who do not have Iron Chests or other supported chest mods installed can still use Sorting Chests without them costing waaay to much.

Just have it always be wood? They did already spend iron on the Iron Chest itself, after all.

But you can then sort more slots for the same price, doesn't seem fair.

Can already do that with a Sorting Interface. You need to balance between fairness and intuitiveness.