Originally Posted by Amexpat

You'll see the Torp airport bus when you come out of the terminal. You buy the ticket from the driver, no need to advance book.

I looked at the Trysilekspressen website and they say that they won't add an extra bus if the scheduled one is full, so you may want to call them at +47 48 16 69 00 to either advance book, or to find out if the bus is likely to be full for your departure.