I need a copy of the receipt for my iphone 11 purchase in March of this year

If you purchases it from Apple online in the U.S., see the following from Viewing & Changing Orders - Shopping Help - Apple

View or Print Invoice

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.