Free shipping for only one carrier (voucher)

[krzyho]

Hi,

 

Prestashop 1.3.1

 

I have question, In my shop I offer free shipping with voucher.
I have created the voucher and set to the category and set price to only be able to be used with that product. Everything works fine but after entering the code all shipping options change to free.

 

In normal shipping (without voucher) i changed "free fees" in cart.php and there i defined carrier which will free.

 

Is there any similar HACK / MOD for free shipping with voucher?

 

Please help, this is very important for me:/

 

Best,

Chris.