I need an invoice for the iPhone I bought a year ago

Subscriptions and Billing - Official Apple Support

You are speaking to end users like yourself on this forum, use the link above to get your invoices from Apple.

The AppleID is your login for Apple Support Communities. If you did not register it with this login, and you bought the phone directly from Apple, they may be able to still look it up by the IMEID number in Settings -> General, or the serial number.

The serial number is often on the back of the phone in fine print. Take a magnifying glass to it and see if anything looks like a combination of letters and numbers and use http://checkcoverage.apple.com/ to see if it ever was registered.

Do not give us the number, as that's your key to support coverage from Apple directly.

Is it listed under your devices? Did you purchase it using your Apple ID? Where did you purchase it?