getpaid-single-invoice-discount-code.php

<?php

add_filter( 'getpaid_invoice_totals_rows', function( $totals, $invoice ) {

    $discount = $invoice->get_discount_code();
    if ( ! empty( $discount ) ) {
        $totals['discount'] .= ' (' . $discount . ')';
    }

    return $totals;
}, 10, 2 );