GST IN CASE OF AIR TRAVEL AGENCY WORKING ON COMMMISSION BASIS

Dear ​Sir/Mam,

I am an air ticketing agent ​under ​Via.com​ and airlines like Goair , Indigo​ . 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 ?​ Will the taxable amount be the commission received from airlines or the travel company (in this case Via.com) ?​

How am i supposed to raise​ tax​ invoice in such case ?

​Do I need to raise GST invoice if I charge extra service fee from customer and if ITC will be applicable in that case ?