Old receipts and invoices can be found under the User Settings -> Billing / Subscriptions section.
1. Under your username on the top-right of the page, click the Settings link.
2. Click the Billing/Subscriptions tab to be taken to the Billing page.
(Or login then click http://keyhole.co/payments/subscriptions.)
3. Once you're on the Billing page, click the View Invoices button.
This will show a pop-up on the page listing all your past invoices. You can view and print invoices here, as well as have it emailed to you.