Somehow I seriously doubt that the vet told you that your snake had mites to make sure you clean your enclosure, and medicated your snake un-necessarily.

Are you sure she said mites, and not some other parasite, because mites aren't internal and detected in fecal floats. Was this an excotics vet, or one that primarily sees dogs and cats?

And btw, store bought would be the first snakes I'd take in for fecal tests, since most pet stores by captive hatched babies in lots, which are more likely to test positive for internal parasites, than captive bred babies.