Bundling

At one stage of developing my 'cassette to CD' website, I came up with a downloadable guide for visitors who wanted to try their hand at converting their own cassettes. Making money from this was a somewhat clumsy process involving 'locking' a PDF file, and providing unlocked versions for those that paid a donation. I was paid anything from 50 cents to $10 via PayPal.
I have re-thought this system recently when I discovered that you can sign up for a bundle scheme that effectively 'bundles' your download with an offer - if your visitor acts on the offer, you can get paid out. You will have seen similar things when downloading free software yourself. It seems to me to be a fairly neat solution, so much so that I think I might dust off my guide and bring it up to date before trying my hand at bundling to raise some of that elusive passive income..