Query related to service tax advertisement commission exemption

Hi All,

I am service provider of advertisement agency work on commission basis. ie i purchase advt space from Mr x and sale to Mr Y that space. Please help me that in which category of service should i registered for it. Is there any exemption in service tax for commission on service tax & rate of tax kya ha.Some one told me that gross amount bill pe take 85 percentage as a exemption and pay service tax on 15 percentage gross amount. is this practice is correct. Please reply