How did your snake get mites?

[Bogertophis]

...Don't know how well a mite can grip onto the plastic packaging....

I don't think they can, but the problem is that bags of substrate usually have air-holes in them, so there actually IS a way in.

I'll add that the few times that I dealt with mites, I was lucky because each time they remained only on the original snake. I had many
more they COULD have gotten into but didn't...there's something to be said for having your cages spread thru-out your house, instead
of all together in one room or worse yet, touching each other.