If I were you I wouldn't be feeling guilty. Its not the design that made me feel guilty. It was knowing that the author deserved to get paid for the work while not paying. So in my case I got something valuable for free. I think it's great to be able to get it, read a bit, then decide its not worth money and not pay if you don't find it valuable. But while this is great, I think there's danger on both sides. The author is giving it for free, allowing you to preview and decide the worth. Awesome. But if you've gotten it already for free and decide its worth your money, like in my case, what's to stop you from never going back to pay. You have to find the link again (which admittedly is super easy to remember -good job) then go through the payment process. There's not much of a barrier to paying after the fact but there still is one. I promised to pay but how will anyone ever know?

So what's the best thing to do? Give it away free and hope the honor system works or force payment and have unhappy customers? I don't see a best answer here, unfortunately.