There isnt a need for a plastic case unless its to ensure the safe transit of your gun. A simple plain cardboard box that is tightly and securely tapped up is the norm. I have never had a problem shipping or receiving something in a simple cardboard box. Just as long as there is not pictures of scary black guns on them it should be fine. Another thing is make sure they use but loads of tape. It drives me mad to see my $500+ gun held in a box by three pieces of scotch tape. I didnt make that up either!!
Also, a plastic case will add at least 1 KG of weight, cardboard box, maybe 200 grams