That Amazon.com review that's influencing your buy? It may not have been written by someone who actually paid for the product. That's because Amazon has an invitation-only program, Vine, for top reviewers—and at least once a month, they get to choose two free products from a list and must review them within 30 days.
Perk for Top Amazon Reviewers: Major Freebies - No. 1 reviewer has gotten thousands in free merchandise