I had a site I was about to let expire a couple of months, but then I decided to use it for a little testing. I researched, and found what seemed to be 4 of the best SEO gigs on Fiverr. I directed each service to a different page on the site. Two of the pages where ranking nowhere at the start, two of them were page 6 or 7. Long Tail Pro KC scores for my keywords ranged between 30-38.
All four services had no positive effect whatsoever. The two sites on page 6 or 7 floated away out of the top 10 pages, never to be seen again!
Like I said, I was planning on ditching the site anyway, so didn't mind dropping the $20 for testing.
You might get lucky and find one that works for you. But I think you are better off looking for a cheap gig in the BST section here. At least you have real reviews here that you can work off. Those Fiverr reviews are a joke.