Whether gst calculates before or after a discount is something you need to check on with the your government offic
if the item is taxable, and the discount is also taxable it works like
100 T
-20 T
80 T
gst will then be calculated on 80
but, if the discount is non taxable
100
+gst
-20
if you are getting less than 80 including gst, that is weird
