Passing over the obvious injustice of the sentence. It strikes me as in many ways beneficial to move the black market drug trade on to the internet. It'll presumably cut the amount of violence between drug gangs over 'turf', because the internet does not have that spatial element. They'll still want to compete with each other, but without access to each others personal space, their competition will have to be non-violent. At that point, you've arguably got to start respecting and rewarding your customers if you want to gain an advantage. Unless you hire mercenary hackers, but let's not make this shit too complicated.
A lot of people might not realise just how easy it is to send drugs through the post. Postmen are delivering parcels all over the place without a clue. And it's a fairly safe way to go about doing it for the buyer. In the event the contents of your parcel is uncovered, the police would have to prove you paid for it, and it wasn't just sent to you at random. Why would they do that? Who knows, but it's possible, so it's a legal defense. I imagine some dealers also like the fact that there doesn't have to be any kind of face to face handover.