If you go to the CarbonPoker site, you can see the standard offer is 'Up to 45% VIP
I think, that if you signed up through the site and specifically asked to be put onto the flat rakeback they would allow you to change, but the sure fire way is of course via an affiliate.
Affiliates typically also offer extras like free coaching videos, strategy articles and various promotions.
You could see what 2+2's Advertising Affiliates have to offer you by posting a Rakeback Request.
Hope this has helped and if you have anymore questions feel free to post here.