First note, that the customers are automatically sent a receipts when the ride is over as long as their email was placed on file. With that said, you can go into 'Settled', and near each ride is a blue receipt button where you can indicate which email or emails to send the receipt to. Or you can go into your 'Status' filter and selected only the 'Completed' phase. Then you can open up a completed ride, 'View Charge' and then have access to their receipts.
Customers can also get their own receipt from the customer app or the website integration on their own. It might help to let them know that they have access to this.