Thats a great tutorial. I've used it for adding IAP in my apps.
It should be easy enough to push a new view onto your navigation controller in the method completeTransaction in IAPHelper.m (I've added the bold part) (something like this...)
- (void)completeTransaction

SKPaymentTransaction *)transaction {
NSLog(@"completeTransaction...");
[self recordTransaction: transaction];
[self provideContent: transaction.payment.productIdentifier];
[[SKPaymentQueue defaultQueue] finishTransaction: transaction];
myNewViewController *myNewView = [[myNewViewController alloc] initWithNibName:@"myNewViewController" bundle:nil];
[self.navigationController pushViewController:myNewView animated:YES];
}