Thank you, Jürgen!
So, just to end this discussion, in case anyone else faces this issue, with help from an ABAP, we've discovered that this is the normal behavior of SAP. The value will only be transfer to field "Freight" if the option for posting unplanned delivery costs on customizing is equal " " or "1". For me, this doesn't make much sense, but that how SAP works.... Thanks for your help, guys.
Regards.