The post office, ah man, I can tell you definitely that the biggest hurdle of all communication - even the fastest, speed-of-light++, smallest messages we can send - is when we reach a certain distances apart in the universe.
For example, assuming we have speed-of-light internet across space stations and colonies, a colony even one light-year away... imagine selling them(ew, selling :P) a new device. To offer the device would take 1 year to receive, to accept the trade would take 1 year going back, the delivery would... well let's just pretend it's blueprint files to 3D-print the device + the software(physical device way longer than 1 year trip). By the time the people offering the device received the go-ahead to trade, the device might be out of date, and the monetary system would like have fluxed and the price would have changed if not the value of currency in general.
So, yes the post office very much could be at the forefront of new ideas for solving this problem alongside every communication.

As for Post Cards, those who deal with high-quality cameras(although I'd assume they'd be everywhere, queue surveillance discussion xP), the creators of space-lighting items, and of course photographers. I'd recommend getting untouched(no editing) pictures in space because of the groups of people out there who think there are no legitimate photos in space. I think legitimate ones get edited often because there is so little to capture and the crafts themselves blend with the shadows so easily(hence why I mention explicit assistance of lighting experts).
Anyway, the post office one will be needed I suppose, since after certain distances we will have a delayed-lag that will be akin to messages-by-horse and worse. Something will have to be done. Universal databases of knowledge need to be synced, etc.

@Nobody At All - I have no idea what you're really after here, but you amuse me. :-)