Okay, that being said, there are a lot of companies and authors looking for book reviews. Product reviews are a different animal and I would not do one of those unless provided with the product.
Here's my personal experience with doing book reviews for Fiverr clients: It was too busy and I could not keep up! Then someone reported me or something and I got shut down. Fiverr shut down my book review gig and won't let me even put REVIEW in any of my gigs anymore. I'm not sure how this highly successful Fiverr seller has gotten away with it for years but see how she words her gigs. Remember that when you are first starting on Fiverr you can't offer the EXTRAS like she is doing ... I believe you can do that after about 30 days. Then it's nice to be able to charge an extra 2 gigs for 24-hour turnaround if you are able.
For now you can charge one gig for the review and another gig for any ebook priced up to $4 and a third gig for any ebook priced up to $8 (Fiverr takes 20% of your sales so keep in mind you get $4 per paid gig and not $5). You keep any overage. You simply purchase the ebook on Amazon, peruse it, speed read, whatever, and give a review. If there are ever any problems with getting paid from a buyer, simply go back to Amazon and request a refund for your ebook purchase.
The seller I referenced above, plus all the others doing this, must be speed readers or have a lot of time on their hands as shut-ins or something???? I can't figure it out and let me know if you can! I wish you success at whatever you try on Fiverr!
*Do not give 24-hour service for free and do not pre-write a review until you have been paid, no matter how big a hurry you are in. There are people/buyers on Fiverr who are looking for free work and will threaten to give you bad feedback if you don't cancel their order and give it to them for free. Trust your instincts on jobs like this.