Would there be a way to set up Paypal to auto email the book to the customer? If I were a paying customer, I'm not sure I would want to wait to get my book. I think the expectation of buying an e-book is to get it instantly...
Now, this is all horribly insecure - if someone discovered the name of their script you can see how easy it'd be to get it to email the ebook to whoever - but for no-cost boot strap solution, it'd do the job initially.
I'd happily lend you a hand getting started with this stuff, my email address is in my profile.