Where can I change email address for billing notifications?

By default invoices are sent to account registration email address, but you can easily change this. Select Notifications from the menu on your right. Then click on the tab System notifications and enter any email address you want:

billing_notifications

Please note that you can enter more than one email address – just make sure you separate them by a comma. Once you enter at least one email address, notifications stop being sent to your registration email.