When you buy something that has to be mailed, emailed, or shipped, you usually get an invoice. An invoice is a document which shows details about your order. It may have various numbers or codes printed on it, including:

• an order number, which identifies your order
• an invoice number, which identifies the invoice itself
• a customer number, which identifies you
• a tracking code, which you can use to find out where your package is while it's being sent
• item numbers for each of the things that you bought