When you place your order, an order confirmation email containing a copy of the order, along with product pricing, shipping cost, and tax (if applicable) will be sent to the email address you provide at checkout. That email and also the shipping confirmation email that is sent when your order ships, both contain a link near the top that says:
"Click here to download your Order receipt PDF"
Simply click on that link to view or print your order receipt/invoice. If you don't have either of those emails on hand and need another copy of your order receipt/invoice, just send us an email (firstname.lastname@example.org). Include the order number and your first and last name to assist us in finding your order. We will email the receipt/invoice to you.