Paying upon approval is ridiculous. I can request a developer write me an app with a buy button that takes the user to my website instead of using Apple's In-App Purchasing. That app would never get approved even though the developer wrote exactly what I asked them to. So I shouldn't pay the developer because Apple denied the app? And no, you shouldn't expect the developer to vet your design before starting development and alerting you of such problems unless you are paying for it. For $800, you are definitely not paying for it.