I refer you to this article posted on Fiverr Academy, that of course, every single Seller on Fiverr has read and committed to memory:
Don’t you love that feeling when you purchase something from a store and receive something extra, that you didn’t expect? It could be a big discount on your next purchase, a free gift or just the amazing after service you received.
I take it that in your case, the Sellers with whom you are working are using the lagniappe as more a bargaining chip for a better Review. I don’t believe that’s how the author of the article meant for his or her advice to be taken.
Many Sellers are simply reacting to the incentives that Fiverr has put in place via the Levels System:
The fact that Fiverr is changing the way Reviews are handled implies that they will make appropriate adjustments to the way Levels are handled, as well.
Hopefully your experience will improve over time.