Example code that shows how to add a new transaction shortcode for use in Event Espresso 4's invoice template

transaction_promotion_surcharge_shortcode.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function register_new_example_transaction_shortcode( $shortcodes, EE_Shortcodes $lib ) {
    if ( $lib instanceof EE_Transaction_Shortcodes ) {
        //Add your shortcode to add as the key, the value should be a description of the shortcode.
        $shortcodes['[RECALCULATED_SURCHARGE_AFTER_PROMOTION]'] = _('This is the recalculated surcharge amount after promotion is applied');
    }
    //Return the shortcodes.
    return $shortcodes;
}
add_filter( 'FHEE__EE_Shortcodes__shortcodes', 'register_new_example_transaction_shortcode', 10, 2 );

function register_new_example_transaction_shortcode_parser( $parsed, $shortcode, $data, $extra_data, EE_Shortcodes $lib ) {
    if ( $lib instanceof EE_Transaction_Shortcodes  && $data->txn instanceof EE_Transaction ) {
        if ( $shortcode == '[RECALCULATED_SURCHARGE_AFTER_PROMOTION]' ) {
            $txn = $data->txn instanceof EE_Transaction ? $data->txn : null;
            if($txn==null){
                return;
            }
            if(!$txn->line_items(array(array('OBJ_type' => 'Promotion')))){
                return;
            }
            $total = $txn->total();
            return number_format($total-($total/1.19), 2);
        }
    }
    //Return the currently parsed content.
    return $parsed;
}
add_filter( 'FHEE__EE_Transaction_Shortcodes__parser_after', 'register_new_example_transaction_shortcode_parser', 10, 5 );