two carrier one invoice help

[pixeljuice]

A few months ago I purchased "product carrier" module which allows multiple costs handling for the shipment of different products however the products belong to the same warehouse.

For example:
For product A the cost of transportation is 10€
For product B transportation charges are free

The purchased module should work properly whether I purchase the two products together or separately.
 
However there seems to be an issue which occurs when I purchase the two products together and try to generate the invoice. Unfortunately "prestashop" generates and invoice for product A and one for product B even if the purchase is made together.
I was hoping I could get only one invoice instead of two.
 
Is possible apply different carrier cost on sigle products?

 

any suggestion?

tks

giorgia