Can any additions be made to the contracted price when ‘Transaction Value’ is acceptable

Yes. Section 15 of the CGST Act, 2017 provides for inclusions to the transaction value (on which GST will be payable). The below are broadly, the inclusions prescribed:
(a) any taxes, duties, cess, fees and charges levied under any law for the time being in force other than the CGST Act, 2017, the SGST Act, 2017, UTGST Act, 2017 and the Goods and Services Tax (Compensation to States) Act, 2017, if charged separately by the supplier;
(b) any amount that the supplier is liable to pay in relation to such supply but which has been incurred by the recipient, but not included in the price;
(c) incidental expenses, including commission and packing, charged by the supplier to the recipient, and any amount charged for anything done by the supplier in respect of the supply until delivery of goods or supply of services;
(d) interest or late fee or penalty for delayed payment of any consideration for any supply; and
(e) subsidies directly linked to the price excluding subsidies provided by the Government.

(ICAI FAQ PUBLICATION 06-09-2017 VALUE OF TAXABLE SUPPLY : FAQ NO.50)