At the end of the sale, you have the option of printing, texting, or emailing a receipt.  Customers who have already opted in to receive digital receipts automatically will get them and you may not see this screen.

When you go to send or print a receipt from transactions or customers profile it will always ask you how you want to send it and require the information.  Even with customers that have opted into receiving automatic receipts.  If you didn't ask for and enter the customer's information during the sale then you will not have this information in your customer profile and you will have to ask them for this information to resend the receipt.