CCBill always send an Email confirmation for subscription following payment for Achat. In the title is a large 18 digit number.
This same number is always shown in the actual email and is called an Order number.
It looks something similar to this 853021734000006895
This is the Subscription ID and the receipt and proof of payment.
You get a confirmation email for each payment made with this ID number on, albeit different each time.
You should keep it and the email safe until your subscription has ended, just in case you have a payment / product query.