A negative externality is the cost borne by parties unrelated to the transaction. In this debate, that party is the environment. I'm confused by your usage of the term.
According to your argument, buying organic food is worse for the environment than buying non-organic food because it costs more. Likewise, buying hydroelectric power would be worse for the environment (have more negative externalities) than buying coal power if it cost more. Do you dispute the logic of this extension of your argument?
According to your argument, buying organic food is worse for the environment than buying non-organic food because it costs more. Likewise, buying hydroelectric power would be worse for the environment (have more negative externalities) than buying coal power if it cost more. Do you dispute the logic of this extension of your argument?