I don't think such a service will ever exist, the legal cannabis industry is way too highly regulated. I hear that some cannabis vending machines that are in development will accept Bitcoin as they are unable to accept credit cards at the machine due to regulations and bureaucracy.
I do not understand your logic in this. What do you mean by 'highly regulated? If they can have cannabis vending machines then why not online services? I think some cannabis dispensaries actually deliver the products to their customers but not sure they would send through the mail. And if you want to buy weed through the mail just go to the darknet market.