I made the mistake of emptying my cartridge and subsequently refilling it later down the line. The move is definitely just to pay for a rental when you are out there. Several of the local Tromso shops carry Mammut canisters, you just have to give them a call to reserve it in advance.
I got my rental at the shop in the mall where
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Shop Name:
Friluftsbutikken AS
Karlsøyvegen 3, 9015 Tromsø, Norway
I ended up going with Interspot, which was the shop inside the mall next to the airport. Please call ahead: