You may wanna try Fastenal, I’ve used them many times with great success.
All you have to do is pallet it, make a crate around it, wrap it, then drop it off at any of their stores, and then they’ll ship it anywhere in the US.
Just find out the nearest Fastenal near your house, and the nearest Fastenal to the destination the kart is going to, call them and they’ll help you with the paperwork. Will probably cut the shipping costs in half, depending where it’s going to. Anyhow, my 2 cents, hope this helps. FastFreddy.