Best way to test if sustainable packaging actually drives purchase intent?

Conjoint is the right call here — it forces real trade-offs so you catch the say-do gap your client is worried about. I'd pair it with a behavioral validation layer, like a simulated shopping task in Qualtrics or a platform like Vividata, to see if eco packaging holds up when price enters the equation. In my experience, sustainable packaging typically lifts intent 8-15% but collapses fast once you add even a 10-15% price premium, so make sure your conjoint includes realistic price levels.