Vaðlaheiðargöng is a paid tunnel in northern Iceland that cuts through the mountains, making travel between Akureyri and Husavik quicker and easier. It's 7.4 km long and uses automatic toll collection and has no toll booths, so you either pay online or get invoiced later.
If you don't pay online total cost is 2.540 isk per trip and will be deducted later from your bank account or in case of car rental you will get billed by the car rental company at the end of your trip.
TIP : you can avoid the tunnel by taking mountain pass paved road 84, which is also a scenic one (note the road can become impassable in the winter so check road conditions on road.is)