edd_email_purchase_receipt()

This function can be used for resending the email purchase receipt for any purchase.

If you ever want to resend the purchase receipt, but need to use your own methods (instead of the built-in option in Downloads > Payment History), you can simply trigger this function:

edd_email_purchase_receipt( $payment_id, false ); 

The first parameter if the ID of the purchase to resend the email for, and the second parameter is whether or not to send a notice to the admin as well (probably should leave this as false.