To get receipts for past orders, go to your account's "Billing" page. Then, in the "Invoices/Receipts" list, click an order number in the "Invoice" column to load it. The receipt can be printed or downloaded as a PDF.
Outstanding invoices and refunds will also show in this list.
Note: Only the past 13 months of receipts are shown. If you need an older receipt, contact support.