Hello Shaylinn-
First I want you to know that the users in this community (You, me and everyone else) do not represent Apple and are not employed by Apple. This is a user to user community and not ➔ Apple Support.
To answer your question, if you have a valid and active Apple Care then you are right that it would cost $99 to repair the back glass of your iPhone Xs Max. According to my best understanding, when the back glass is cracked and needs to be replaced then Apple just simply replaces the entire iPhone Xs Max with a refurbished iPhone. The inventory of the refurbished iPhones is not declared on Apple web pages and that's why you didn't see any when you investigated.
Note that if your iPhone Xs Max is not still covered by Apple Care then it will cost $599 to replace it. For more information click here ➔ iPhone Repair - Official Apple Support.
Axel F.
Axel F.