If it's cheap to make it's because there's little environmental impact.
It's not a perfect one-to-one, but it's really close.
> practices like "buy three and return two which we'll throw away" will no longer make financial sense
But you are not changing consumer behavior! People will still buy things and return them, and those returned items are worthless, i.e. not sellable. The manufacturer can try to donate them, but no one will want them, or they will just store them forever in cheap warehouses that will burn down every once in a while, or suffer water damage.
Or perhaps you want there to be artificial shortages in stores, so there's just barely enough to buy because the store will be fearful of overbuying something they will have to keep on their shelf permanently?
Please, try to think of the results of your suggestions. You keep thinking only of your goals and not what will actually happen.
Too cheap. And this raises the cost, not in materials but in overall lifecycle cost of producing an item. That's the point. The cost of overproduction is too low, by raising it, bad practices like "buy three and return two which we'll throw away" will no longer make financial sense. The excess shipping, storage, etc. won't happen because it won't be a viable business strategy.