If at any time you feel my feedback isn't helping, I will refund the full amount. If I don't find time to review your startup, I will also refund the full amount. I'm not an expert on startups. The more sites and ideas I'm exposed to, the better I can get at finding holes in my work. Charging will likely make you and me more committed to the review.

A bit self serving.. Isn't it? How about you do it for them for free in the beginning and if they think the advice is very good, then they pay you $40 instead of what you are proposing?

If you are looking to do an experiment, rejected people that are down and vulnerable are not the people for your little experiment.