You discount isn't based on fixed %. Look: 1 -> 13/15 = 0,866% 2 -> 25/30 = 0,833% 3 -> 40/45 = 0,888% 4 -> 50/60 = 0,833% 5 -> 60/75 = 0,800% if you want to calculate the final amount based on the quantity of products according to the picture you sent, you should do something like th...