Post-sales discounts: contingent incentives to dealers constitute consideration for services and attract GST when charged. Clarifies that post sale discounts without dealer obligations relate to the original supply and may be excluded from the supplier's value of supply if statutory conditions are met. Discounts contingent on dealer activities are consideration for services, attracting GST charged by the dealer with corresponding input tax credit to the supplier. Supplier funded reductions to induce lower customer prices are consideration to the dealer and must be included in the dealer's value of supply; customers' input tax credit is limited to tax actually paid. If discounts cannot be excluded, suppliers may issue financial/commercial credit notes but cannot reduce original tax liability; dealers need not reverse input tax where they pay the reduced value adjusted by such credit notes.