Amazon announces they are now providing Amazon Prime members with a 20% discount on preordered videogames, saying this applies retroactively to existing preorders. This goes along with their existing perks of a price guarantee and release date delivery, and they explain all the details in this FAQ. One of the answers notes that this actually does not require committing to a game before reviews are in, as the discount extends two weeks after release:

Yes, Prime members get at least 20% off list price on new physical video games during pre-order and two weeks after launch. If the game is discounted less than 20%, Prime members get the full 20% off at checkout. If the game is already discounted more than 20%, Prime customers get the lower price.