The following at line 100 in sales/customer_payments.php does not work.
display_note(print_document_link($payment_no."-".$trans_type, _("&Email This Receipt"), true, ST_CUSTPAYMENT, false, "printlink", "", 1),1);
The following too does not work:
submenu_print(_("&Email This Receipt"), ST_CUSTPAYMENT, $payment_no, null, 1);
The above alone does not work even with necessary changes made to use $email in reporting.inc and rep112.php.
Try the consolidated changes for the bleeding edge attached and provide feedback.