Hi PK Andreassen,
What is the PSA module? Is this something you customized / bought as an add on or do you refer to the Project Management and accounting module?
Have you tried recording a transaction in a project expense journal?
Those journals provide you the possibility to record quantities such as 50 kilometers and a cost price per quantity, which will then be used for posting.
Would be great if you could test and update us here about the outcome.
Many thanks,
Ludwig