If you local track has a message board try to find someone nearby otherwise it’s not worth the shipping as mentioned. You can find most 40mm axles for around 75 used.
I dont agree that the benefit of a 50 is because its softer. Most are 2mm thick and I have some 40s that are 1.75 mm. Not to mention to increased stiffness from the diameter. When testing both the 50 seemed to have more grip and stability without binding the kart.