Apple will send you the invoice by mail 2-3 days after the delivery of your iPhone.
You can also print a copy of your invoice by visiting this Apple website Online Self Service.
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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 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.
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copied from: Viewing & Changing Orders - Shopping Help - Apple