Prestashop 1.6.0.8 custom settings of free shipping value calculation

Hello,

I need a custom setting on Prestashop 1.6.0.8 free shipping calculation on amount.

I need some products on which I set specific shipping costs to override free shipping based on amount.
So if I set a specific shipping cost on product item, it's override free shipping and show on cart
such cost but if some others items reach the "free shipping amount" for those items no other shipping
costs are added to the cart.

I give you an example:

Settings shipping with free fee for orders over 500€

1) I've found a solution on which a single product also over 500€ keep shipping fee, If I set on
such items "specific shipping cost" so it's override free shipping and show on cart specific shipping
fee. I've found it here and modified cart.php on /classes:

http://www.prestashop.com/forums/topic/202426-additional-shipping-cost-overides-free-shipping/

So my first target it's ok, but my problem comes when on such cart another item that is used to calculate
free shipping over 500€ is added.

For example

1. Product of 600€ + 20€ specific shipping cost (it's not calculated to reach the amount of 500€)
+
2. Product of 700€ with standard shiping costs so it should be free, but it's come with shipping cost.

So cart became wrongly as below:

Products:
600€+700€

Shipping:
20€ (specific shipping cost of item 1)+
10€ (standard shipping cost of item 2)

My target on cart is:

Products:
600€+700€

Shipping:
20€ (specific shipping cost of item 1)
nothing more when other item/s reach 500€ and they do not have "specific shipping costs"
or standard costs for item/s that do not reach 500€

So finally on items on which I set "specific shipping costs" are not calculated for "free shipping on amount".

Please let me know if it clear and you can solve it, tks

Kind regards

Davide Redeghieri