Enabling additional shipping cost per quantity

[tonake]

Hello. I've been configuring my shop to work with my country's ridiculous post delivery system/its prices (which are calculated according to BOTH the weight and the price), and during the course of it I realized that the easiest way I can do that is by enabling additional shipping cost at the delivery tab of each product. However, there's one problem: now this cost is added to every carrier that isn't free, even though it's apparently only supposed to be applied to carriers that have taxes enabled. 

 

The thing is that I want local courier delivery to have a fixed price, without this additional shipping cost being added to it. This carrier option has taxes disabled, but still has the additional shipping cost added. Does anyone know why/what to do about it? Thanks in advance.