Follow these steps to display the last five orders for the selected customer:

1. Import the RPC utility in the /static/src/js/pos_demo.js file by adding the following code

var rpc = require('web.rpc');

2. Add the following code to the /static/src/js/pos_demo.js file; this will add a new action button to fetch and display the information about the last five orders when a user clicks on the button:

var lastOrders = screens.ActionButtonWidget.extend({
    template: 'LastOrders',
    // Add step 3 and 4 here
});

screens.define_action_button({
    'name': 'last_orders',
    'widget': lastOrders
});

3. Add the button_click function in the lastOrders widget to manage button clicks:

button_click: function () {
    var self = this;
    var order = this.pos.get_order();
    if (order.attributes.client) {
        var domain = [['partner_id', '=', order.attributes.client.id]];
        rpc.query({
            model: 'pos.order',
            method: 'search_read',
            args: [domain, ['name', 'amount_total', 'cu']],
            kwargs: { limit: 5 },
        }).then(function (orders) {
            if(orders.length > 0){
                var order_list = _.map(orders, function (o) {
                    return { 'label': _.str.sprintf("%s - TOTAL: %s", o.name, o.amount_total) };
                 });
                self.show_order_list(order_list);
            } else {
                self.show_error('No previous order found for the customer');
            }
        });
    } else {
        this.show_error('Please select the customer');
    }
},

4. Add methods to show the order list and errors:

show_order_list: function(list) {
    this.gui.show_popup('selection', {
        'title': 'Please select a reward',
        'list': list,
        'confirm': function (reward) {
            order.apply_reward(reward);
        },
    });
},
show_error: function (message){
    this.gui.show_popup('error', {
        title: "Warning",
        body: message,
    });
}

5. Add the QWeb template for the button in the /static/src/xml/pos_demo.xml file:

<t t-name="LastOrders">
 <div class='control-button'>
 <i class='fa fa-shopping-cart' /> Last orders
 </div>
 </t>

Update the pos_demo module to apply the changes. After that, you will be able to see the Last Orders button above the keyboard panel. When this button is clicked, a popup will be displayed with the order information: