I actually tried Reptile Relief "back in the day." It didn't seem to do anything other than kill off the mites that yo could see. A little while later, and the eggs that were there must have hatched, because mites returned.
Provent-a-mite is definately the way to go. Used as directed, it is safe. It is not something to be sprayed right onto the snake; as per the can, you clean the cage, spray it onto the substrate, and let it dry before putting the snake and hide spots back in. Water bowls need to be removed during treatment as well; no PAM in your snake's drinkie!
I would recommend using paper sowels or newsprint as a sbustrate when mites are being killed. It greatly enables you to see the little buggers better than a loose substrate.