Best way to test consumer willingness to pay for biodegradable packaging?

Conjoint is definitely the right instinct here — specifically choice-based conjoint (CBC) works great for this. I'd layer in a Van Westendorp Price Sensitivity Meter as a quick sanity check, since sustainability messaging can inflate stated WTP versus actual behavior. Tools like Sawtooth or even Qualtrics conjoint modules handle this well with around 300–400 respondents.