As I said in a previous post I got mine posted to me by the Oamtc (Which I think is an Austrian version of the AA or ADAC.)
They were extremely helpful and only took a couple of emails to sort it out and paid by BACS, the sticker was posted and arrived in a few days.
The only problem is I don’t know who I originally contacted to start the ball rolling.
I did notice that the German trucks at the circuit had either a German emissions sticker or an Austrian one not both, not sure if there is a bit of cross border understanding,
Would be good as the way it’s going we’ll need automated trucks as you won’t be able to see out of the window after it’s been covered with toll boxes and emission stickers, 
I believe the French now (or will soon) require one to go into various cities.