Shiping cost depend on items quantity in cart

[bja]

I need Prestashop to calculate shipping cost depend on items quantity in cart. If there is more than one item in cart, i want the shipping cost to be 50% of sum of all shipping costs.
All of my products have Additional shipping cost.

I modyfied the cart module, but when i proceed to order page, the shipping cost is still full amount.

My code in blockcart:

if($nbTotalProducts>1) {
$shipping_cost_float = $shipping_cost_float/2;
$shipping_cost = $shipping_cost/2;
}

How can i store new shipping cost for all modules/controllers?