I always tell new hires the same thing, "Keep your feet moving and your mouth shut, and you can work here as long as you want." (Not that I always followed my own advice, especially the "mouth shut" part. But after 10 years, I am pretty much mute at work.)
Preloading (loading, sorting, delivering etc.) is a marathon, not a sprint.
If someone insinuates that you are working "too hard", they are not necessarily trying to get you to be a "slacker". They may just be warning you that in your eagerness to please and work "hard", you set yourself up for unreasonable expectations. Your only reward for doing (X) number of units of work per hour or shift, will be an "attaboy" and a request to to (X) + 10%. When you accomplish this new benchmark, you will be asked to do (X + 10%) + 10%. This is an endless cycle that will continue until you quit/retire/get injured/handle all packages in the building yourself.
Work Safe.
If you get hurt, ultimately, the only one who is going to really give a "hoot" is you.
Many new hires, especially young and male, tend to want to show off their "muscles". That little twinge in your back as you huck a 85lb box by yourself, may not seem to bother a 22 y.o. body. But, by 50, that one long-ago decision might keep you from lifting your grandchildren or being able to hike more than a mile without pain. UPS (or any other co./union/co-worker, etc.) will not be there to commiserate with you.
Respect Seniority.
Wait a year or two before judging what you think is going on. More than one new-hire who took me for a "lazy geezer" when they started, now rely on me for advice, information, and support when dealing with workplace issues.
"The machinery and I are permanent, the packages and supervisors are just passing through."
You will have supervisors that you get along with and some that you won't. Enjoy the good and outlast the bad. In my experience, the supervisors are basically impotent (this assumes you show up on time and follow methods). The only thing that really bothers them is if you rub that fact in their face. Don't. Remember, we succeed in spite of the supervisors, not because of them.
Bad Supervisors.
Channel the spirits of Forrest Gump and Cool Hand Luke. Be good natured, say "okay". Then do what you know works best, is most efficient, and safe. When they return and get upset that you are not doing exactly as they asked, say, "Oh, okay." Repeat as necessary. Bad supervisors assume you are lazy/stupid no matter what, so why fight it or get upset. You will not change their mind. Do NOT try to convince them that your way makes more sense. To a bad supervisor it is absolutely impossible for a teamster to make sense.
UPS is a great Company.
The UPS brand was created by thousands of dedicated, hard-working people, most of whom were teamsters. Don't let management "BS" stop you from taking pride in the work we accomplish each day. This job requires the ultimate in self-confidence. If YOU know you did a good job and did your best, that is the best you can hope for (and a paycheck). Being employed as an hourly union worker within a giant publicly-held corporate machine has its positives, satisfying the stock-holders will never be one of them. Accept that.