Ingredients for Sending Email/Updating Database "before page load"

I want to wait to post the data into the tables and send email confirmation until the payment has been successful -- the "before page load" trigger of the checkout_success page.
I have left the page redirect BLANK for both of the server behaviors.

What are the active components of this checkout_success page?
What are the rules that need to be followed so this page works correctly?
What bindings, in general, need to be on this page.

This process has been "hit and miss" with me and I end up wasting so much time;
-redoing and redoing -trying things slightly different,
-searching these forums time after time, but never finding a comprehensive solution outlined.

I don't necessarily just want to get my current project working correctly, I want a sort of checklist that I can use each time to make certain that my shopping process is working the way it is supposed to.

Thank you for your help.