If you check out the Review Program Guidelines
, down towards the bottom it talks about Buy-out assignments.
Basically, if there's a toy you really want to review, but it already has a bajillion and a half reviews on it, you can request to do it as a buy-out. In exchange for 30% off the product prices, you'll write a review. The review is the same as your normal monthly assignment. A difference though, is you can request more than one buy out at once (I don't know a limit) and get them all at 30% off. You'll still have to turn in the reviews in the normal time frame though.
Hope that helps!