Follow these steps to add a custom credit template:

1. Add a template with service information at views/templates.xml:

<odoo>
<template id="no_credit_info" name="No credit info">
    <section class="jumbotron text-center bg-primary">
        <div class="container pb32 pt32">
            <h1 class="jumbotron-heading">Library ISBN</h1>
            <p class="lead text-muted">
                Get full book information with cover image just by the ISBN number.
            </p>
            <span class="badge badge-warning" style="font-size: 30px;">
                20% Off
            </span>
        </div>
    </section>
    <div class="container">
        <div class="row">
            <div class="col">
                <div class="card mb-3">
                    <div class="card-header">
                        <i class="fa fa-database"/> Large books database
                    </div>
                    <div class="card-body">
                        <p class="card-text">
                            We have largest book databse. It contains more
                            then 2500000+ books.
                        </p>
                    </div>
                </div>
            </div>
            <div class="col">
                <div class="card mb-3">
                    <div class="card-header">
                        <i class="fa fa-image"/>
                        With cover image
                    </div>
                    <div class="card-body">
                        <p class="card-text">
                            More then 95% of our books having high quality
                            book cover images.
                        </p>
                    </div>
                </div>
            </div>
        </div>
    </div>
</template>
</odoo>

2. Add a template to __manifest__.py:

...
   'data': [
        'security/ir.model.access.csv',
        'views/book_info_views.xml',
        'data/books_data.xml',
        'views/res_config_settings.xml',
        'views/templates.xml'
    ]
...

3. Add a template reference to iap.charge at controllers/main.py:

...
with iap.charge(request.env, service_key, account_token, credits_to_reserve, credit_template='iap_isbn_service.no_credit_info'):
    data = request.env['book.info'].sudo()._books_data_by_isbn(isbn_number)
    if data['status'] == 'not found':
        raise Exception('Book not found')

Update the module to apply the changes. After the update, you will see a credit popup if all of the customer's credit is consumed: