Or in other words, I can send the information entered into the form to the credit card processor and get the "This transaction has been approved." message back. That part is working and done.
Now all I have to do is send the people (and only the people) who have been approved to the "Choose a username and password" page.
The "Choose a username and password page" is working and done too and it will take them through the rest of the signup process.
But at this point I don't have a way to send people who have paid from the page with the form on it to the choose a username and password page.