Hi, not having used paypal, but having tied up to other payment gateways they all return via POST or GET a transaction number and a success code , so your thankyou page should really do nothing without checking for those variables?
MVC is the current buzz in web application architectures. It comes from event-driven desktop application design and doesn't fit into web application design very well. But luckily nobody really knows what MVC means, so we can call our presentation layer separation mechanism MVC and move on. (Rasmus Lerdorf)