I just decided to purchase rfactor after playing the demo.
So if im trying to actually purchase it with the launcher, I wont come to the payment section because of
Unavailable: Login required. it appears right after im logging in, dont know how to fix.
I used this link to purchase directly in a browser. I believe this is what is displayed in the demo purchase interface. It might work better for you or help you to troubleshoot the problem. I don't know why they don't have a direct purchase link on their site.