Pricing customers not consistently

I want to play an online game. I have a strict NAT Type which limits this funcitonality. i have friends who had the same issue. When they contacted the ISP they created a public IP and said when it is for gaming, it is at no additional cost. They want to charge me a R100 a month to create the public IP. I don't agree with their method of business where some customers get charged for additional services and others don't