A number of Apple employees have told AppleInsider that the back glass isn't covered under screen repair and is instead qualified as "other damage," the fee for which is $99. This is likely because removing the glass back is markedly more difficult than swapping out a screen. Unlike the front glass, the back glass is glued in really well, requiring much more effort to remove. AppleCare+ allows for two incidents of accidental damage, after which your repair price jumps up to $349 for the iPhone 8 and $399 for the Plus for anything other than a screen repair.
So be careful with that iPhone 8. Between higher AppleCare+ costs and higher damage repair fees, that new phone could turn out to be much pricier than you bargained for.