For customers on the Business Plan, the Admin can view and download Invoices on the web application. 

To do this, login with the admin credentials and access Account Settings. On the left menu, you will be able to see 'Billing':

The Billing page contains a section 'Billing History'. You can click on the 'View Invoices' option and you will get a pop up with the past invoices. From here you can download the invoice in pdf format.