Please, please be extremely careful when cartopping thin sheets of plywood. It would be a far better plan to have somebody bring it home for you in a truck or van. This stuff can catch the wind and break very easily if it isn't clamped or tied down securely. Sometimes the use of other pieces of lumber as backers can accelerate the force and provide a fulcrum for the forces involved. I had a thin piece of luan break and go flying into traffic on a busy highway even though I used clamps and a couple of long two by fours to secure it. Thankfully it didn't hit anyone or cause an accident. The near miss scared the living daylights out of me, and I suppose that it made a very strong impression on me because I am a lawyer who practices in the area of personal injury.