pos/pos_invoice_send_mail/static/src/xml/pos.xml

<?xml version="1.0" encoding="UTF-8"?>
<templates id="template" xml:space="preserve">

    <t t-extend="PaymentScreenWidget">
        <t t-jquery="t[t-if='widget.pos.config.module_account'] t[t-if='widget.pos.get_order()']" t-operation="append">
            <t t-if="widget.pos.config.iface_invoice_mail and widget.pos.config.invoice_mail_template_id">
                <div t-attf-class='button js_send_mail #{ widget.pos.get_order().is_to_send_mail() ? "highlight" : ""} '>
                    <i class='fa fa-envelope-open-o' /> Send Invoice by Email
                </div>
            </t>
        </t>
    </t>

</templates>