I hope that he passes his drug test, it seems that you have provided a "catalyst" by your firmness in standing your ground. this is real improvement. He may kick up a fuss but his back is up to the wall if he loses his internet.
it is exhausting trying to outthink him with the mail and the postal service, I'm sure. I think that you are getting so much control back though and I'm glad the therapy for you is continuing.