I recently bought a dog collar on Amazon. A week or so later, I got a large envelope in the mail, with "Amazon Early Reviewer Program, 2646 Rainer Ave S St 1020, Seattle, WA 98144" as the return address.
Inside was a piece of paper with a code to "activate my $20 gift card" by leaving a 5-star review of the dog collar, photographing the review, and emailing it to a very non-Amazon email. It also instructed me that "for security reasons", I should not include or attach this letter to my review, nor leave a negative review without emailing them first. All this still purports to be from Amazon proper with the Amazon logo on the paper as well.
This is clearly a scam from the seller to buy 5-star reviews, and their product has plenty of what look to be paid reviews to me as a result. I tried very hard to find any place to report this scheme, which goes past review buying to include impersonating Amazon itself, and mail fraud. I emailed several Amazon contacts, used contact forms on their site, and contact forms I could find on the seller side as well. I attached photos of the letter.
The only response I got was a form letter telling me that if I saw a fake review, I should click the report button under that review.
This is just one example. Almost every product I've bought in the past year that came from an individual seller included some kind of bribe for 5-star reviews, from gift cards to whole "free products every month" programs.