I haven't seen one of those, or sent one, in any of my conscious memory. Everything that comes in the post now-a-days is printed matter and it's either complete crap or bills. I don't think that people actually handwrite any longer. I know that unless I concentrate very hard, my handwriting is utterly incomprehensible. My cursive script is utterly abominable. I suppose it's a lost art and we can lament it's passing, but in this day and age people value the message over the way the message is sent.
There are still some things that demand handwritten messages, but none of them should be delivered by post. Mostly love letters and poetry. Those should be hand-delivered. I can't think of any other reason why you would send a message over the post in a handwritten way other than to perhaps stroke some sense of nostalgia.
The world has moved on.
