That's the usual practice. You could say that "even though the review was requested I'm giving my honest opinion".

If you're in the US, you should probably state that in the review to stay on the good side of the FTC. If you're outside of the US and your country doesn't require you to disclose that fact, then it's just a matter of how transparent you want to be. If it was me, I would probably disclose that there was an incentive for the review.

I've never done an incentive review before but from my experience looking at other sites they usually have a disclaimer saying that the review was sponsored somewhere. Either at the beginning or some text at the very end.