You need cargo racks. Just buy whatever cargo they want and they say 'thanks, now lets continue to our destination'. When you deliver then to the destination, they leave with the cargo. Usually more trouble than it's worth. My approach is to have a fast ship with a lot of jump range, so I can often be done with the mission before they ask anything. Also it's safe to ignore them once. If they want to talk to the manager, well I am the manager.