There are currently two operators that offer daily bus tours to Þórsmörk. Below you can book bus tickets with both companies depending on the schedule that fits you.
The end of Fimmvörðurháls trail is Básar, the Volcano Huts are in Húsadalur which is 6km away from Básar. The tickets need to be bought online with more than 24-hours notice.The seats are sometimes sold out, so book well in advance!