Best way to test if sustainable packaging actually influences purchase decisions?

Conjoint is the right instinct — it forces real trade-offs instead of letting people just virtue-signal their eco values. We ran a CBC study for a CPG client last year with packaging type as one attribute alongside price and brand, and the stated vs. revealed preference gap was huge (people claimed 40% more willingness-to-pay than conjoint showed). If budget is tight, even a simple monadic price sensitivity test with eco vs. standard packaging can expose the gap pretty quickly.