Flipbook Printer actually underwent an unusual process. I sold the commercial rights to it many years ago and struct an arrangement with the buyer that the free version would remain available, and in fact not even need a license key -- but that printouts would say "not for commercial use".
So the answer is that you don't need a license key to use the program and all of it's features indefinitely -- but that you can't use it for commercial use or remove that text.
As always, I'm happy to refund any donation if you like. I apologize for any confusion/frustration.
ps. I don't believe they are still using the software any more, so you might try to track them down and enquire about the rights to use it commercially.