I've been testing turn.js v3 for personal projects and am seriously considering buying it for production use in a business; it's the only thing out there for dynamically-rendered flipbooks. Sorry to hear about your troubles. At that level of revenue, I'd seriously consider charging using something like Stripe or Braintree. Your target audience (developers) won't consider typing a CC number as such a huge inconvenience.
Another note - releasing the 4th release's source code under something like the Affero GPL (or a similar noncommercial license) could drive adoption of that version, since many people like to "try before they buy" - and would like to do so with the most feature-filled version.
Another note - releasing the 4th release's source code under something like the Affero GPL (or a similar noncommercial license) could drive adoption of that version, since many people like to "try before they buy" - and would like to do so with the most feature-filled version.