Depending on you Transsmart contract you will have rates available for calculation of the freight cost.
When using the carrier selection function this freight cost can be added to the sales line.
in a lot of scenarios you can consider this freight cost being expected cost because you are in the start of your delivery process.
Lot's of reasons can be the cause of having a transportorder ready for the booking to transsmart which will have different information on item, packages and may be a different carrier.
Posting the transportorder will cause the actual cost invoiced by the carrier. This will not update the sales line whent retrieved.