SpaceGremlin (mac alternative to WinDirStat) has a similar thing, where some features only work in the independent "SpaceGremlinPro" version downloaded from their site. However, they do some cool stuff with licensing - you can point it to the app store paid/installed version, and it detects the license and unlocks.
If you're worried about people not trusting payment to you, might be worth seeing if you could implement this, so anyone who bought on the app store can still access the full feature set. Cuts you out 30% like, but better than nothing maybe.