I think that everyone on here has said it pretty much on the spot. As a sup, I hate writting people up for misloads, even though higher up management requests that I do. I have had to cover for sick preloaders before, and I have misloaded packages. It is part of the job in my opinion. Now, someone who misloads more than three every single day is taking it a little far. But I would still work with them on how to avoid it in the future. We have changed the names on the SPA label before (actually we do it quite frequently). Letters and Numbers start looking the same after 3-4 hours.
My advice, put it back on them. Make them show you, or explain to you, how it should be done. Also, if you are working so fast that you cannot do your job safely, then you have a problem. One of UPS's biggest things is being safe. If it isn't safe, then stop the belt (if you can) and get to the point where you can work safely. If they argue with that, then you need different management. Something you cant control, for sure. But maybe the Union can help you.
One more thing. Hang in there. Although it isnt always fun, it can be rewarding in the end. If you get the chance to drive (if thats what you want to do), you'll sure like the pay! We are a small center though, so we have a lot of fun, most often.