"Well...princess...you see, Pinkie threw one of her 'special' parties and Applejack brought one of her 'special' beverages...I got drunk and tried to fax my butt to Canterlot. Then at the last second I realized just what I use for a fax machine..."
Or maybe part of the flame is magic. Like, the part of the flame that's hotter than a certain temperature sends messages, but the edges just burn stuff. Like two thirds of the pages of the book disappeared, for instance, so for the rest of that episode I was wondering if/when Princess Celestia would send them back.
Oh, also: Tickets. The tickets go through Spike as well. Now, it's possible those are also enchanted, but I think it makes much more sense if it's Spike.
It's not so much selective as dependent on (probably) temperature. If the dragonfire is above (to put an arbitrary number on it) say 700 Kelvin, whatever is inside the flame has enough energy that the teleportation can happen.
Oh, also, if it's the scrolls , why don't we see them getting thrown into a fireplace instead?
SEEING HOW WE'VE A CREATURE WITH MANY BODY PARTS, A PONY WHO CAN SCARE A COCKATRICE WITH THE "STARE", A PINK PONY WHO CAN POP OUT OF NOWHERE & PULL THINGS OUT OF HAMMERSPACE I THINK SPIKE HAVING 2 KINDS OF FLAMES WOULDN'T BE THE SILLY THING. XD.
Heck, I am an adult, and I enjoy adult humor, but I make my adult humor dependent on the the reader. In this series of comics, I left it an open question if Celestia is hitting on Twilight, or just Trolling her... or both. That way, people get out of the comic what their mind expects out of the comic, which may or may not be what they want to read.