pos/pos_mail_receipt/static/src/xml/pos.xml

<?xml version="1.0" encoding="UTF-8"?>

<templates id="template" xml:space="preserve">

    <t t-extend="ReceiptScreenWidget">
        <t t-jquery=".button.print" t-operation="after">
            <div class="button email">
                <i class='fa fa-at'></i> Mail Receipt
            </div>
        </t>
    </t>

</templates>