[mothered]

although I can't really see there being two possible weights which wouldn't arouse suspicion.

If It's the entire contents (box & Item/accessories), the shipping (gross) weight must be taken Into account.

As for the Item alone, It's the net weight that's of relevance.

I don't want to return anything (box accessories) since I plan to resell it.

Then as per above, It's the shipping/gross weight of the box and Its contents. Be sure to calculate the weight of the dry Ice, by allowing enough time for storage and transit.