You may want to approach the employees and ask if they have another job, before coming to UPS. At least, you can start a conversation, and give them the opportunity to give a reason for their body odor.
That in itself, may make them think, why would you ask that type of question and they might figure out on their own they need to address their personal hygiene issues.
Unfortunately, the issue is not in the contract, is not a UPS safety issue, so someone has to make the employee aware of their offensive odor. More often than not, a UPS management person will take an employee aside, and discuss the delicate (no pun intended) situation.
I can't understand why anyone would not practice good personal hygiene, as there is no personal satisfaction or self respect in being offensive to others, be it at work or anywhere else.