[mothered]

Does it work if I opened the big package box ?

It's the laptop box/packaging that's of relevance.

I know what the sealed box method is, but I realize I haven't thought about it since it looks... too beautiful to be real, somehow ?

The method has one of the highest success rates.

Like they really won't check if the article is inside its own box ?

If there's no signs of tampering whatsoever, It will not give them reason to check the return.

And the 800g difference left would be like, dry ice ?

The box will not be opened on return, so add anything you like.