If you are connected to an accounting system you must request this setting via: email@example.com if it's not already set up for you.
Customers can be connected to Rates in Godo Property. These rates can be in ISK/EUR for accounting.
Here is how you register a customer in a certain currency.
Find the "Currency" line within "Customer" under Settings -> Guest Management.
You can select "ISK" or "EUR" within the red box as shown below.
Note: "Code" within the blue window is the connection between "Customers" and "Rates" within the system. See below: