Cheapest is by bus, but if they are worth a lot of money you should try using a shipping company. I never shipped just parts, I did however ship whole bikes. A few years back I bought a bike but it was across the country from me and I didn't have the ability to get down there and bring the beauty myself (though it would have been one hell of a ride). So I ended up using a shipping company (You can check them here: https://cheapmotorcycleshipping.com/) In general it a good idea because they were very professional they didn't damage anything. And they were fairly fast so I felt like it was an overall pleasant experience. I am not sure if they ship parts but I think they might so give them a go.
