Transaction value determines taxable supply, including specified charges and linked subsidies; discounts excluded if invoiced and ITC reversed. The Act makes transaction value the primary basis for supply valuation where parties are not related and price is sole consideration, and specifies that value includes separately charged taxes and fees (other than the GST Acts), amounts paid by the recipient on the supplier's behalf, incidental expenses (commission, packing), interest or penalties for late payment, and subsidies directly linked to price; discounts are excluded if invoiced at or before supply or if post-supply contractual discounts are linked to invoices and attributable input tax credit is reversed.