What kind of "truck" does the shipper think you have? Maybe they are thinking that you have a regular pickup truck, are just planning to have the box containing the cladding in the bed (which would make it very succeptible to theft), and don't want to be held responsible.

If the new cladding can be removed from the box and put in the VX's cargo area for transport, everything else would be the same as far as what the transporter was responsible for.