Your current and previous invoices (from the 1st. of august 2014), from Bókun for the monthly fee are all kept in your Bókun system.
To find your invoices select the "Cog wheel in top right corner" then you select “Account & Billing” from the side bar:
Account & Billing, gives you overview over the subscription package you are subscribed to, the amount of user accounts, as well as the invoicing information.
Below the overview, you have a list with all you previous and current invoice and their payment status. You can see your invoice by choosing the name of the month and you can also download it as a pdf.