There are a number of discs that have checkout discounts right now; not sure if that's part of the upcoming Prime Days promotion? The discount appears on the item page, under the list of "editions" (usually to the lower right of the item picture, in green font). It's $4.30 for this title, also for Monolith Monsters, and I've seen other discounts of varying amounts on a number of discs (Shazam! 3D has a $7 checkout discount). Note that these discounts will only appear when you checkout. They will not appear as discounts on the item page.