What if you replaced the monitor with a pack of gum?
What if you replaced the pack of gum with an envelope containing one of
your knowingly false and fraudulent US federal tax returns? That has about as much relevance as the question you just asked, i.e., none at all.
In my mind [the OP's story] is ... unfathomable[.]
The main problem here is that both you and Guses haven't actually engaged in a reasonable analysis of this situation. Among the plethora of defects found in your overly simplistic posts is that you've both fixated specifically on the weight of the object, while ignoring the size of the box even though the original post specifically said that "the box was bigger ... than expected".
Let's keep in mind that the original poster ("OP") purchased the monitor online and it may have been shipped from a warehouse to the local store, which means that the monitor might have been in at least two nested boxes (the manufacturer packaging and the box added for the warehouse shipping), and each of those two boxes might have contained significant padding and mostly open space. As a result, we might have been talking here about a very large box indeed. One of the challenges in transporting such a large box isn't just lifting the weight, but also how to best place one's arm around the box to carry it, especially if a person has shorter arms than you do. The mass distribution within the box is also uneven, but the monitor might be positioned in such a way within the box that the optimal way to position the mass doesn't mesh well with the contours of a comfortable way to hold the box.
The difficulty of loading such a box onto the bus isn't just the physical lifting, but also the challenge of how to best maneuver a large object through a relatively small doorway up into the bus. Keep in mind that the pathway up into the bus probably involves having to walk up several steps, which means the OP had to figure out how to best maneuver the box so that she could get her arms around it and lift it onto the bus, but also had to do it in such a way that she could continue to walk up the steps of the bus with the box.
Furthermore, the situation is also made very stressful by the fact that the other passengers on the bus just want to get where they are going, and aren't interested in waiting around. In such a situation, both the bus driver and other passengers might start to show that they are annoyed. Even if they don't say it, you can see it on their faces, and it's discouraging and can add pressure to an already-stressful situation. (The OP doesn't specifically say anything about this, but another source of stress is that for a woman carrying a purse on her shoulder, it could start to slip off in this situation of trying to lift a large box, and this can be stressful because there is a risk of, say, one's wallet or phone falling out while trying to get onto the bus.)
My point here is that whether or not the OP had the physical ability to lift the box, there was likely a fair amount of technical skill required to do so, and executing on that level of skill in a stressful situation can be difficult for people who don't have a lot of experience with this kind of thing.
More to the point, the OP was under no obligation to go into great deal about why her original plan didn't work out, and it was frankly irrelevant to her story. Your fixation on analysing what, in your mind, she did wrong serves no purpose other than to insult her and attempt to make her feel bad. I doubt you've been successful in that latter objective because of the transparent inanity of your posts, but you've made a valiant effort.