Best way to test whether sustainable packaging actually drives purchase?

Run a conjoint analysis or even a simple A/B split test on your e-commerce product pages if that's an option — swap packaging imagery and track conversion rates over 2-3 weeks. If you need attitudinal data fast, a quick 200-person survey with a willingness-to-pay question comparing eco vs. standard packaging will give you something concrete to show the client in under two weeks.