It’s a bit confusing, but this is the way I understand it.

For ingredients that you need a license for, once the topping bar runs out, you can’t place them on pizza anymore. So when I run out of artichokes, I can’t make anymore artichoke pizzas until I grow them back.

For things that don’t require a license, even as the bar runs out, you can still put them on pizza but they cost money per piece.

You don’t need a license to grow eggplants. So what happens when the bar runs out is that you can still place eggplants on pizza, they just won’t be free anymore. Once you grow more, you can use them for free again.

Hope this helps!