Your proof of purchase is both emailed to you, and stored safely in your account.
FOR PURCHASES MADE ONLINE
Your Online Order Confirmation email serves as both your proof of purchase, and as a Tax Invoice.
If you are unable to locate your Online Order Confirmation, your purchase history can be accessed by logging in to your online Account. If you checked out as a guest you can retrospectively create an account with the email addressed used, and your purchase history will be visible to you there.