Don't use the appDelegate. Check the if statement in a ViewController. The reason why is not working for you, is because your calling this in the AppDelegate. Where are you calling this code? DidFinishLaunching method?
What I would do is: When purchased: Create BOOL key and call this when the purchase is completed. Then you can still use the appDelegate to check with NSUserDefaults.
[Flurry startSession:FLURRY_ID];//FLURRY_ID is the #define @"flurry_adID"
[FlurryAds initialize:self.window.rootViewController];