Gst valuation, rate

I am an IATA agent but purcase the tickets from other IATA agents- Now my question is how will i charge gst on ticket sold to customer

if sold at markup and if sold at discount and i am paying gst @ 18% on commission received from  them

and i am selling overseas packages for which i will buy tickets, Hotel, Visa, Food and others from other travel agents and i am adding markup selling to customer as package. can i Pay Gst @ 18% on Markup?

can you guide me please