Hi,

I think you must have missed some screen. There are two things involved in this. 

1. Creating an account (Student Account) on College Board site. This is just an account in which you will get all notifications and everything related to SAT. This is very important. 

2. SAT Registration - This is a different process in which you register for SAT. Once you start this process, college board will ask you for the Fee Payment. If there is a confusion. Check the URL below with screenshots.

https://www.testprepkart.com/sat/blog-single.php?id=615/3-Ways-to-Register-For-SAT-Subject-Test-

Also, if you have not paid yet, you can go ahead and try registerting for SAT again and this time you might get the payment window. If the payment is processed, you will get everything aligned in your Students Account. 

Hope this will help you. Let us know if this worked.