Re-declaring a function in a child theme

Solution:

In your check for function existence, there’s a space behind the function name. !function_exists('jem_render_buy_fee ')

Without this space it should work fine if the order af calling code is correct.

First Call:

function jem_render_buy_fee() {
    $fee=(ea_get_option('order_commission_buyer'))?ea_get_option('order_commission_buyer'):'';
    if($fee){
        ?>
        <div class="jem-commission-fee">
            <span><?php _e($fee.'% GST inclusive', 'themes'); ?></span>
        </div>
        <?php
    }
}

Then call:

<?php
if (!function_exists('jem_render_buy_fee')) { //Space removed from 'jem_render_buy_fee '
    function jem_render_buy_fee() {
        $fee=(ea_get_option('order_commission_buyer'))?ea_get_option('order_commission_buyer'):'';
        if ($fee): ?>
            <div class="jem-commission-fee">
                <span><?php _e($fee.'% commission fee included', 'themes'); ?></span>
            </div>
        <?php endif;
    }
}