mattmiller and su6oxone are both right - the /checkout/ is the shortcut way of doing it if you just want to enter a URL, while su6oxone's way is how to do it via the clickety clickety.
It seems you've figured it out, but for for future reference when folks look at this thread - you'll need to make sure the derive task reruns in order to create mp3's from your new FLACs. Otherwise they'll just sit there underived...