Every purchase, subscription and top-up is recorded in one place, so you can pull any document later for your records.
Your transaction history
Open Billing and go to your Transaction history - a single feed of your activity you can filter by All, Purchases, Subscriptions, Top-ups and Refunds.
Documents you can download
From the feed you can View invoice, View receipt, and download the license document for a purchase. Your active licenses are also listed under Licenses.