The lightest material that you can buy that breath/vent sweat out are the best but have the most durability that can survive the metal edge that may come across your leg like a dull butter knife with small chance of jagged edge that may stick out. But, also, avoid dark or "black" based material clothing. In UPS, you are required to wear work boots. So, your goal is to find the lightest work boot that sold from your shoes store place. Think of it, you are a package handler. You are going to move a lot. So, generally, for situational purpose, you are wearing a light clothing that offers the best agility and dexterity in return.
Forget about the "looks". You should feel confident and have your wits about yourself. When people get busy, they don't have lot of thought about you unless you happen to be a model looks like which might earn you the wave and smile. But, come on, UPS work is going to be hellish in some case, and an angel is all it take to brings glimmer of hope to make it through.