There are different ways to buy a ticket:

Buy single ticket and 24-hours ticket with farte-app before boarding or onboard the bus. Farte-app is available for downloading in AppleStore or GooglePlay. You may also use cash or credit card.

Buy 30-days tickets and travel money before boarding online at web store. You need a travel card to buy these tickets. Travel-card you get on the yellow buses or at the Narvesen stores in Skien, Porsgrunn and Stathelle/Brotorvet.

You can buy travel card onboard the buses or at Narvesen in Grenland. See info here.