Sir,
According to Section 15 (2) (a) of CGST Act, 2017
The value of supply shall include–––
(a) any taxes, duties, cesses, fees and charges levied under any law for the time being in force other than this Act, the State Goods and Services Tax Act, the Union Territory Goods and Services Tax Act and the Goods and Services Tax (Compensation to States) Act, if charged separately by the supplier. Any tax collected except CGST, SGST or UTGST is to be included in the value of supply.
However, in my opinion, the customs duty recovered by the Customs Broker (Agent) is not towards providing customs broker service. It is independent of the service provided/supplied. Hence customs duty collected by customs broker from their client need not be included for calculating Rs.10 Crore turnover,