Sharing My Expertise for ECom & Payment Gateway

Recently many people approached mostly from Market research companies and budding founders for giving an hour or two to get my feedback/review for the product they are building.

I honestly give them insight and share feedback that they can value a lot from.

Usually I do it for free , giving my time in weekends but I realised people get paid for this as one of the company offered me Amazon vouchers for it.

Should I continue doing it for free or charge them?