If an invoice has been paid, a receipt should have automatically been sent to the customer, comprising of a customized version of the message found under Customer Receipt Email template on the Email page.
If you'd like to re-send a customer receipt for any reason, you can do so by opening the desired paid invoice and clicking Review & Send in the top-right.
This will cause a modal to opening, containing the Customer Receipt Email message. Review and edit any details if you'd like, and then click Send. The customer will then receive the receipt in email form.
Note: Be sure that you're only performing this action on a paid invoice. If you do this on an unpaid invoice or quote, it will cause the invoice or quote to be re-sent to the customer.