Hey i'm not a business consultant but i know that there can be 2 discount forms..one is related to the payment method ( "sconto pagamento" in italian)..imagine you pay in cash at the checkout in a shop...there you can get a discount and this is calculated after VAT and in these terms you pay all ammount for a service/product before invoice!..the purpose is to reduce cash flow income latency from customers...i mean Shop owner can receive all the money in one time payment.
The other is a discount related to the particular marketing conditions ( sconto cassa ) ...so when you want to compete in a market or simple reduce stock of old products etc etc...this kind of discount must be calculated before tax ( on "imponibile"...product total cost - Discount + VAT)
ciao