When you book you will pay 15% prior to arrival, with the remaining 85% being paid upon your arrival.
When you've selected a vehicle, there will be a breakdown of the amounts on the right-hand side of the vehicle page (like below).
Northbound processes the payment in ISK (Icelandic Króna). You can change the display amount in the top menu. Amounts in other currencies are estimates that are calculated with exchange rates which are updated every hour.
After adding a vehicle to the cart you can see a detailed price summary with all the payment information.