Valid point. So basically, all dice are cargo. Spice cargo pieces usually can't be plated. Shipwright specifically plates only "found cargo pieces" meaning enemy drops and floating cargo. I guess the wording on that isn't quite clear enough.
Yup, intended to work like that. It only triggers when you first see the dice. Making all 3s plated (and others too when he is leveled) which you can then gamble a bit and reroll to be higher.
