I've only had to use a trucking company twice. The first time was a 48"x48" to Key West.
The most recent shipment was a triptych consisting of a center 30"x40" and two side pieces 30"x24"which went to California.
My husband built the wooden crates. The paintings were wrapped in sheet plastic. The crates were lined with foam rubber.
I shopped around and chose Old Dominion Freight. It may have been cheapest because they are out of Winchester, VA not far from where I lived. I bought insurance separately. The cost of the materials, insurance and freight was around $250, if I remember right.
A lot of times, I can combine the delivery of a painting with a trip I'm making. I have a Honda Element which is remarkable for how large a painting it can haul. ;^)