Partially built then use plenty of fiberfill to suspend the build and lock it in place to prevent shifting. As far as pricing you have to compare with different carriers. You will need a good 3” of dead space to prevent shipping damage. More on that below.
I would not put any “Fragile” or “Handle With Care” labels. The times I did the boxes arrived like a truck ran over them. I know people that worked at FedEx and USPS and said those that are marked as such are usually targeted by jerks along the loading process. My last commission was a huge 1/32 scale Tomcat that was very carefully packed inside a large thick heavy duty cardboard box. I decided to add two warning labels to the box which was a big no no. It arrived with one crushed corner and two golf sized holes poked on the sides. Thanks to my careful packing there was 0 damage to the Tomcat. The dead space around the model did the trick.