People making money using Facebook groups to market Amazon products with promo codes up to 99% off or free with 5-star review. How?!

Hope the title says it all. But I've been part of groups on Facebook that pretty much the admin is plastering coupon promo deals up to 99% off with a code or review in exchange for free stuff on Amazon. I believe they make a commission on the products they end up selling. Their codes don't last very long and if you wait too long, the amazon discount code expires.

Can anyone tell me how they do this?

Did you get anywhere on this?

Following, interested to know too.

Do you know any source of a daily feed of active Amazon promo codes?