I am also selling paid support and add ons for my open-source Rails engine for Stripe named Payola, which very recently gained support for subscriptions.
Edit: You can read a free preview here: https://www.masteringmodernpayments.com/read
Very cool. if you're helping people earn money, they will pay :)
Is the high-end team options selling? Also what it the split between the mid-price and the lowest price?
I'm trying to figure out the pricing for my book so it (a) is dont-need-to-think-about-it-cheap, (b) doesn't devalue the book (if too cheap readers won't trust), and (c) interacts nicely with Amazon who are undercutting me on price. Any tips?
 http://minireference.com/static/newsite/ cf. http://minireference.com
Low and mid tiers have sold approximately the same number of copies, which means the mid tier brings in about double the revenue.
Not sure what to tell you about Amazon. I don't sell there because I like taking 97% of my revenue instead of 30%, but for a math textbook maybe it would make sense. Do you market to math tutors? It seems like if you could get tutors on board they would be able to easily sell to their students.
I'm thinking a good partner would be university departments who offer tutoring for free---the incentives would be there since they'll need to hire less tutors.
To setup and collect payments world wide outside those countries is a pain in the ass. Any book on that (or good services) deserve money & success.