Everybody gets a digital version—as in, I'll be distributing ebook formats from the website as soon as the book is finished.
Sending a book costs about $10 for the book, $2-5 for shipping, and days of my time to orchestrate. It'd decimate more than half of a $25 donation. I do see the appeal of perks, but I think the trade-off is unattractive in this case.