I suspect some users might prefer a paid advanced version online instead of having to download a desktop app. You could either try a monthly subscription of $1 or a one time fee the same as your downloaded version.

To see which wording of "BuyNow" or "free trail" etc works best you should do some A/B testing using something like Google Web Optimizer. Hopefully it will speed up the process of finding the correct wording/strategy/layout.