It is my understanding that this PC luggage allotment, and the 2*PC came from a thing called the Warsaw treaty, and is valid on transatlantic and continuing flights. 2 pieces of 32 kg each.

There is a standard fee for overage. There is a formula for individual PC (32kg), which I cannot find, but is in the neighborhood of $100 Italy-IAD.

There is a formula, which I have conviently forgotten, for overweight luggage(exceeding 32kg). It is related to weight units times some full class fare, and is very big bucks. I think around $7 per pound.

I have on more than one occasion carried back a number of 32kg packages, from Italy, and paid the around $100 fee. I do not know if it is calculated in euro or dollars, but I do know it is per under 32kg piece, and far and away the cheapest way to move cargo fast.