The problem with this is that I'm using two different companies to fulfill the books and the PDFs, and there's no easy way to tell Gumroad to give a discount to people who bought it from Ookoodook (or vice versa). I could give out a coupon code, but then it's almost guaranteed that it will get out onto the internet and a lot of people will buy the book at whatever discount. Neither system supports individualized automatically generated coupon codes. Also, Ookoodook charges your card when you make the purchase (because that is a large part of how I pay for the print run) while Gumroad doesn't charge until the PDF files are released, which could lead to situations where people buy the book from one place, get the discount code, order the book from the other at a discount and then cancel the first order.
Basically, I would need to have unified fulfillment in order to pull this off without a lot of people taking unfair advantage of it, and I don't.
That was just a typo. It's fixed now, at 160 pages.