[mothered]

but even if I'm supposed to be in a rush wouldn't it be pretty unbelievable that I placed to weights and a book inside the box so it can match the weight of the computer ?

Replace It with an old/used PC.

As such, It's easily mistaken.

I thought about another story, I'd like to have your opinion... I don't have children, but if they found out the thing, I was thinking about a story like, I made this laptop as a gift for the birthday of my son, and he changed his mind, hence the return, but he's the one who made the "exchange" so he could secretly keep the laptop and sell it or I don't know. Like he's the one who resealed it and all, and I apologize and blabla and please can you accept that I return you the laptop ? Something like that, I would appear very surprised and all to learn that the laptop is not inside the box.

Good thought.

There are so many different approaches that can be used. My previous post serves as a general example, which should be manipulated to one's circumstances/environment.