AMAZON: give positive reviews on line, get free stuff.
You don't necessary have to give positive reviews. I've been among the top 1000 reviewers for a decade or so. Just due to my ranking, companies started offering me stuff, but they didn't seem to care (or even notice) that my few thousand reviews ranged liberally from one to five stars, and that if something was crap I wasn't afraid to call it crap. Indeed, even after I accepted free products in exchange for an honest review, I have found the bulk of these to be Chinese crap, at best merely satisfactory for their purpose, and usually horrible, and I've said so in my review. And yet, those same companies continue to offer me the next product they are trying to develop hype for.
I am aware that a lot of reviewers who accept free stuff give invariably positive reviews to keep the goods flowing, but I really don't think that is necessary if your reviewer ranking is squarely in the top 1000. You'll continue to receive free stuff even if you are brutally honest.