Impact Quantity from attribute value on product page

[fx1]

Hi,

 

On the product page I have a combination of attributes in a drop down list with some numbers like 100, 200, 500 , 3000 to choose from,

Each one are defined to discount the UNIT price lower depending of the selected number. On the product page it works, the selected number is impacting the UNIT price correctly BUT I wish that "Quantity" default Prestashop field below the price get multiplied by the selected number as well.

 

Any idea ?

 

Cheers.