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I Googled it and pretty much it says that “open up the box” means open the box fully/completely and “open the box” means open the box just enough to see what’s inside.
To be honest, it still doesn’t make any sense because when you ask someone to close a door, you say: “close the door” you don’t say “close up the door” because it is intuitive to think that of course you need to completely close the door.
Now, my hypothesis is that “open up” is more like a very expressive term mainly used to command/emphasize to open something that at the same time will be almost impossible to close. The differences between the two still don’t click in my brain, so if you have examples, analogies, or explanations to share, I’d really appreciate it a lot.
Planning to buy a gaming laptop and saw a lot of good ones that said open box, certified by geek squad