How to get an invoice of a device bought on line?

See the following from Viewing & Changing Orders - Shopping Help - Apple

Once an item is shipped or picked up, Apple will create an invoice for your order. To view or print your invoice, go to your Order Listing(Opens in a new window) page and select an item to go to the order details. Then, select View Invoice at the top of your order details page to access your invoice.

If you need help viewing or printing your invoice, speak with an Apple Specialist at 1‑800‑MY‑APPLE.

For items from the on-line store go to apple.com, scroll to the bottom, and click on Apple Store Account.