I've seen people offer their guarantee and say, if you're not happy you can have your money back, PLUS you can keep the ebook -- or software, whatever. They know there's no guarantee anyone's going to delete the copy from their computer, so they make it easy on themselves. Just give permission to keep!
Others simply state since nothing can actually be returned, I hope you'll understand no refunds will be given. Booklocker.com has this policy, or used to. I use this policy, too. I'm torn between wanting to be fair to my buyers, but avoid being ripped off myself. I've also seen some of the internet experts use this policy, stating, at this low price I won't be giving any refunds . . . Just be upfront about it.
I'd also say, it depends on what you're selling. If it's a high priced product you're saying will change my life in a huge way in 3 hours, then there ought to be a refund in it if nothing's different in three hours! A big exaggeration here, but you get the point.
As almost everyone says, though, not that many people ask for refunds. I've bought a few things that weren't as wonderful as they were hyped up to be. Keep them around as lessons on how not to do a report on whatever subject it was on!
Anita