Hello, first and foremost, you cannot buy reviews or offer anything in returnma, azon has very advanced systems for detecting fake reviews, and if they catch you, they can suspend your account or even take legal action, what does work is using the official Request a Review button that amazon has in your seller dashboard., tis method is completely legal, and you can only use it once per order, within a specific timeframe after delivery, you can also include inserts in your product listings asking for reviews, but without offering gifts or discounts in return.
Another legal option is the amazon vine program, with vine, you send free units to reviewers selected by Amazon, and they publish their honest opinions, without you being able to influence what they say, the problem is that it's only available if you have a registered trademark in brand registry, and it costs money for each product you register, vine reviewers tend to be more critical than the average customer, so make sure your product is good before sending it.