If your account is integrated with a payment processor, and there is a payment run via Credit Card or ACH, the contact(s) on the event will automatically receive a receipt. If you make a manual payment in the payments tab (i.e. Check, Credit Card in Store, Cash etc), you will have the option of accessing that receipt by selecting the gear button on the payment and choosing receipt. This will give you a URL to find the receipt or send it to the guest.