1. After adding the products to the current sale click on Pay Bill at the bottom of the billing screen

2. Enter the email address in the Customer Email Address field before completing the sale

3.After entering the e-mail address, select the payment method and complete the sale. The soft copy of the bill will be mailed to the entered e-mail address

Note: The soft copy of the bill will be auto-mailed if you have added the customer to the sale. No need to re-enter the email address

If you forgot to enter the e-mail address else you want to resend the bill copy, you can refer the following article How to resend receipts via email?