It affects bartering with vendors, but does it affect buy prices for book return terminals? I'm using a character who can have up to 17 charisma, but said charisma doesn't seem to affect the prices displayed on the terminal (50 tokens for a fusion core, about 15 for a telephone, etc., seems unchanged).
Charisma affects only other people/creatures. You can't charm a vending machine to sell you something for cheaper via charismatic means since it's inanimate. The Wiki page makes no mention of this for the book return terminals, nor does the Charisma page. From my experience, I've witness the same as you have.
Looking at this from an in real life perspective, you might be able to charm someone into selling you something for cheaper. It makes sense that the book return terminal isn't influenced by such means, since they can't experience feelings.