This article explains how to view your invoices in ProBackup and how to receive them by email.
Viewing your invoices
Go to Settings > Billing.
Scroll down to Invoice History.
Click "Open invoice" to view an invoice in your browser.
Receiving invoices by email
You can have invoices sent automatically to an email address of your choice.
Go to Settings > Billing.
Scroll down to Billing info and click "Update information".
Enter the desired email address in the Billing email field.
Click "Save".