You can purchase a Flume Pro licence from flumeapp.com. You will receive a Flume Pro licence code after checkout, which can be used to unlock Flume Pro's features - no separate download of Flume is necessary.
Note: Flume Pro licences are one-time payments and not a subscription.
Mac App Store Version
Please note that Flume for Mac is no longer available on the Mac App Store as of April 6th 2017. If you have already purchased Flume Pro via the Mac App Store, you must migrate your App Store purchase into a Flume Pro licence.