The technique demonstrated in this recipe will update the receipt that is printed from the Point of Sale receipt screen. If you are using POS Box or IOT Box, this technique will not update the receipt printed from it. It used the XML format to print a receipt to ESC/POS-compatible receipt printers. To modify the receipt, you will need to extend the XmlReceipt template instead of PosTicket. See the following example; this will update the receipt printed from the POS Box or IOT Box:

    <t t-extend="XmlReceipt">
        <t t-jquery='.after-footer' t-operation='after'>
            <div>
                <t t-esc="saved_amount"/>
                you saved in this order
            </div>
        </t>
    </t>

Odoo uses the XML-ESC/POS Python library to generate receipts. Refer to https://github.com/fvdsn/py-xml-escpos to learn about the supported XML structure.