UV doesn't travel through solid objects such as envelopes. I think a little bit may get in through windows, but not a significant amount. It's light, although a frequency of light we can't see. Full-spectrum lamps, like the ones used for plants, give off UV radiation/light.

UV also kills the tuberculosis bug, and UV lamps are sometimes used in hospitals. They could use the lamps in Post Offices and mail rooms in the areas where mail was stored in bulk, but it would only kill the bacteria that got out of the envelopes and wasn't buried at the bottom of a pile. I also don't know how long it takes to kill the bacteria - seconds, minutes...days?