OK, stupid question, feel free to point out a FAQ URL if I'm really off-base, but...

Assume I set up an account with Plimus or Paypal. Assume I have hosting to download demos of a game which can be unlocked to a full version with a key.

My question is: how do I connect completing a purchase transaction to getting said key? I figure it's CGI and all that on my server (or elsewhere), but I'm not having much luck getting more specific than that...