Will the Customs duty paid by Customs House Agent on behalf of the client also be required to be included in the transaction value

Under the Service tax law, the aforementioned expenses were treated as
reimbursements as ‘pure agent’ and are hence was not liable to service tax. Similar treatment exists in case of pure agents under the GST law as well. In terms of Rule 33 of the CGST Rules, the expenditure/ costs incurred by the supplier acting as a pure agent of the recipient, on fulfilment of prescribed conditions, be excluded from the value of supply.

(ICAI FAQ PUBLICATIONS 06-09-2017 VALUE OF TAXABLE SUPPLY: FAQ NO. 60)