I recently joined Ejunkie for exactly the same purpose - selling an ebook using an activation key (don't suppose you're using Desktop Author by any chance?). It's been quite a long and painful road, and although I'm not quite ready to launch I'm getting close.
To test the system, which combines Ejunkie and Paypal, I set the ebook price at 1p (sterling).
Testing has shown up a couple of little quirks, especially with Paypal, so let me know how you get on. I'd be happy to exchange experiences.