To access your receipts, go to the Subscription page, scroll down to Billing History, and click Download All.
To download a receipt for a specific transaction, click the PDF file icon next to it. You can also include additional information in your receipts by clicking Add Details:
If you made your purchase via the Apple App Store, you can locate your receipt by following the instructions outlined in this article: View your purchase history for the App Store
If your subscription has expired, please use the form on this page to request your receipt from Grammarly Support: I need a receipt