Lets's look at where things will be located:
(click on the links to see sample scripts)

mysite.com
|
|-reg
| |
| |-key_log.txt
| |-reg_stack.rev
| |-button.html This is no longer necessary
| |-success.html
| |-secrets_file.txt
|
|
|-cgi-bin
| |
| |-myreg1.cgi
| |-myIPN.cgi
| |-myPDT.cgi
| |-myliburl.rev
| |-revolution
|
|-img
etc.

What these files are:

• key_log.txt
  customer license keys and other information is here--also the 'pass-through' variable!
• reg_stack.rev
  the stack your program will load from the internet to begin the process. It will generate a 'pass-through' variable and the license key.
• button.html (Now obsolete)
  generic file your cgi will use to create a button --it will include the 'pass-through' variable.
• success.html
  generic file your cgi will use to present a "Thank you" to the customer upon completion.
• secrets_file.txt
  You can store you latest pricing information here as well as your paypal email adress.
• myreg1.cgi
  This cgi will update the key log with the the user's key and tracking "pass through" variable.
• myIPN.cgi
  The name is "Instant Payment Notification", but it's not instant. It's most useful for when customers pay with echecks or some means that requires some time to confirm. Use this for your inventory, record keeping, etc. Probably a good place to email customers about their order.
• myPDT.cgi
  This is where customers will be directed after paypal. Use this to generate your "Thank you.html" If the payment is completed, you could also use this to show their license key.
• myliburl.rev
  you need this to POST
• revolution
  everyone's favorite cgi engine!

Important: the 'pass-through' variable allows you to track the customer throughout the process. I generate a random number for this in the reg_stack.rev and send it to myreg.cgi
When the button is created, the pass-through variable will be included. So you don't have to include any sensitive info in the [very readable] button script!