Customers enjoy buying eco-friendly products as they feel they can contribute to the bigger picture with their consumer choices. Such purchases make them feel good about themselves. On the other hand, bio, eco-friendly or supporting charities products tend to be more expensive (they are more expensive to produce or companies put a higher price point on them to use that some customers are ready to pay a premium price for such products). To advertise them to the general public (and not only those customers who are ready to pay premium) and boost their sales you can run a promotion on them. Moreover, the promotion can bring you more positive PR and free advertising as environmentally friendly products get more press coverage than usual and their promotions may be considered news-worthy.