[koolba]

Can I disinfect other things like mail or a cardboard box with a UV light?

Alternatively, would leaving things under the sun have a similar effect and, if so, how long would it take?

[throwaway2048]

UV-C is basically entirely blocked by the ozone layer and doesn't reach the surface of the earth.

Its extremely dangerous, and can cause eye and skin damage with under 60 seconds of exposure, it is absolutely something you do not want to be anywhere around when active.

[nfpfish]

UVC is safest actually. Short wavelength is absorbed by dead skin cells. Never gets to live.