Discount per quantity on variable product doesn’t work

Hi,
I’m using free version of plugin. I’m in this situation: I’ve specific quantity discount different for each products (for example: shampoo x (price 10$): from 1 to 5 unit, 10% discount (unit price 9$), from 6 to 20 unit, 18% (unit price: 8,2$)). With simple product, plugin work fine but only with simple product. The problem is that when I select 2 pieces of variant 1 and 15 pieces of variant 2, the products are displayed in the cart individually, with the price of the single variation * quantity. (row 1: variant 1, price $ 9, quantity 2; row 2: variant 2, price 8.2 $, quantity 15; total quantity: 17, total price: $ 141). This is incorrect because it should be: total quantity 17, so I apply the 18% discount and get $ 139.4 (not apply discount individually for all variations but on total items of specific product). How can I solve this problem? Can it be solved in the premium version? if yes, how?