GST in case of Air Ticketing Agent working on commission basis

I am an air ticketing agent of all domestic airline companies in India and also an agent under online travel companies like Travelboutique, Via.com , yatra, akbar travels etc. I get commission for selling air tickets to customers who avail my services. Air tickets already include GST of 5%. As an agent do I have to pay any separate GST ? If yes, then how do I calculate the taxable value on which I have to pay the GST and what will be the % of GST payable ? How am i supposed to raise invoice in such case ?