All past receipts/invoices for the monthly subscription charge are available directly within the account. They are available by logging in and and navigating to Options >> Plan Info >> Billing history. All receipts can be printed or exported to PDF.
Here is what the billing history page looks like.
If you need to have the monthly receipt emailed to someone other than the primary account owner there is an option in the right column of this page to specify additional email addresses. Whenever the card on file is charged the primary account owner and these email addresses will receive a monthly receipt via email. This can be useful if you need a bookkeeper or accounting department to receive a copy of the monthly receipt automatically.