You can send your reader a receipt directly from your Contacts. In order to generate an automated receipt:
Look up the user in Contacts, enter the email address in the search bar, and then click on the record to see the expanded details:
Under the Activity section towards the bottom of the user's profile, click the overflow menu (three dots icon), and select Email receipt. This will trigger an email to the associated user's email address that will include a receipt for their payment.