Taxable value for GST for Travel agent reselling a package or Air ticket

If a travel agent is selling a tour package he has taken from a B2B bulk supplier of packages who has already levied 5% GST on the total package, then on what value should the travel agent levy 5% GST when billing his end retail customer. If 5% is again levied on the total value of package taken from B2B supplier, wont it be double taxation. Same query for Travel agent selling air ticket taken not directly from airline but from a bulk IATA ticketing agent who has already levied 5% or 18% GST on the base fare. Earlier 15% Service tax was being paid only on the commission of the agent.