So you want a Humble Bundle, but for books? Maybe they could call it the "Humble Book Bundle"! And maybe they could give it a humblebundle.com URL too!
There is no need to use sarcasm, you can just inform the person you are replying to without trying to belittle them
The parent's tone doesn't come across as belittling to me - it seems more like a playful, joking form of sarcasm - although I guess tone is difficult to decipher on the web. Nevertheless, there's no harm in a little humour, and I prefer to give parent the benefit of the doubt.
The Humble Book Bundle has only appeared fairly rarely (maybe that's changed now?), and tends to stick to "geeky" books, unfortunately.
Thanks, I've bought things from several Humble Bundles but I'd never heard of this one.