Unfortunately for the receiver, anything can be opened by a customs agency. It’s just a risk with international transfer. Could have legally happened even if you went and picked it up yourself, though chances are less.
"sealed and graded"? Am I understanding this correctly... people buy video games still in the package and leave them in the package as collectibles like they do with toys?
Just the other day I was thinking this would be a great way to smuggle contraband, but that assumes anyone outside the 'scene' knows what the container signifies.