Sounds like a customer that thinks the work revolves around just them. Was always told that customers have a pickup window. Simply talk with your supervisor about the problem. Always was that you get there in that window, if they are not ready when you get there, you take what they have and let them know you can not guarantee them a second pickup before they close. Train your customers to understand they have one pickup, guanateed to be made in the window that they have been assigned. If they ask you to come back they need to make accomodations to have someone there when you return. They can have sombody wait, or leave the packages where the driver can get to them. See what your supervisor can do about this problem, don't do anything differant until you discuss it with him, and he discusses it with them.