[Apekuz]

The shipment units are probably how their freight service determines the shipping size, for instance when I import my goods some cargo services count by predetermined sizes called units. However this is just an assumption, I'm not really sure what it means for them. Also, I don't think you'll be charged extra as long as it's under free shipment.

As to answer your final question, I once ordered several albums and singles from them at one go and they all arrived neatly packed into one box, as opposed to the usual parcel.