This is such an unbelievable non issue!
UPS implemented the dress code rules and regulations for one reason & one reason only. You are a projection of their company, look sharp, clean shaven (both men & women), with a bit of pride in your company. They probably had to bring these into focus, because a handful of idiots couldn't grasp this & had to show off their individuality, and when the people who pay them to do their job, stopped them..... Oh My God!
Use common sense, is your appearance worthy of handing your wife, teenage daughter, or grandmother a package. Do you feel they need to call in security when you stroll onto the lot? I ran into the local DHL chap a few days ago at a stop. His uniform looked as though it hadn't been laundered for a month, with a dark ring around the collar. His face had missed contact with the actual razor scraping his skin for at least a week, silver duct tape was the theme for his footwear. I was indeed fortunate to be up wind of him for certain. Upon exiting the building, his truck was pulled nose into a parking spot, & it too looked as though it had taken third at the fair demolition derby, un washed, running, with a waft of overpowering rap music descending on all in the 50' radius to it.
If this guy showed at my house & my daughter was the recepient of the package, I don't think I'd approve much, as to regard to any other circumstance, I would question his presence at my house, but the soiled uniform & truck allow him access I guess.
If your necklace is hidden under your shirt, and only at the right angle can a hint of chain be seen, and management has a problem with it, you should talk to them as it sounds like they have dealt with an idiot, and are running the no exception rule. If you are trying to spice up the unifrom with a little bling bling, leave it in your locker.