RoRo ships usually charge based on the vehicle's volume (length x width x height, calculated in CBM) in tiers; container shipping has a fixed compartment price. We will calculate which method is the most economical for the current sailing.

We will directly check the registration certificate (the big green book) and count the number of changes.

No, this is a serious customs violation. Complete vehicles (HS Code 8703) and auto parts (HS Code 8708, etc.) are subject to completely different import tax rates. Combining them on one invoice could lead to the invoice being returned by customs and may expose you to tax fraud accusations at the destination port.