The entire field of economics is based on the idea that if you increase the price of something, people will buy less of it. If we increase the demand for plastic, that increases demand for fossil fuels (since fossil fuels are needed to make plastic), which makes the price go up, which means people will buy a bit less fossil fuel to burn in their vehicles etc.

Just search for "gas prices" on Google News and you'll see that people are very concerned about the price of gas. Much more than most commodities, actually.