If I give you less than optimal service, I'm not going to volunteer my consent or a review token or however this works. If I give you it at the start of our interaction, you can hold me hostage as so many try to.
I recognise the problem but a receipt or proof of payment should be enough. And that's already more than what's currently required.
Who's paying for all the other stuff? If you're funding this service conglomerate why not just have paid reviewers?