So put in the description 1 case = 12 pcs and price by case eg. 1 case = £100, 2 cases = £180
Then on the next one put 1 case = 20 pcs and price as needed, 1 case = £150, 2 cases = £280
I don’t see the need to have it display the quantities in the dropdown (12, 24,26) when it can display the quantity of cases (1, 2, 3) and say how many items are in a case in the item description.
Though if you want to pay extra for an app go for it if your happy.