The Receipt records are automatically generated when payment is received through any process that allows for payment. This should also automatically trigger a receipt email to be sent to the customer. However, you may run into a situation where you need to manually send the receipt email or resend it.
Open the Receipt Record
Select Send Receipt Email and Click Add
Click Ok in the pop up window to confirm Application of workflow.
Confirm email sent
Return to the regarding record and navigate to activities. Switch the view to all and you should now see the sent email record.