In this part, we adjust the previous code to show the Preview page, optionally.

class PreviewRedirect(RedirectView):

    def get_redirect_url(self, **kwargs):
        self.request.session['payment_method'] = self.request.POST.get('payment_method')
        return reverse('checkout:preview')

from django.urls import path
from .views import *

urlpatterns = [
    path("preview_redirect/", PreviewRedirect.as_view(), name="preview_redirect"),
]

...
{% block payment_details %}
    {% block payment_details_content %}
        
            <form action="{% url 'preview_redirect' %}" method="post">
                {% csrf_token %}
                <fieldset>
                    <input type="radio" name="payment_method" value="oscar_paypal" required> Oscar PayPal <br> 
                    <input type="radio" name="payment_method" value="stripe" required> Stripe <br> 
                    <input type="radio" name="payment_method" value="paypal" required> Paypal <br> 
                    
                
                    <button type="submit">Continue</button>
                </fieldset>
            </form>


    {% endblock payment_details_content %}
{% endblock payment_details %}

{% block place_order %}


    <div class="form-group clearfix">
        <div class="row">
            <div class="col-sm-4 offset-sm-8">
                <br>
                {% if request.session.payment_method == "oscar_paypal" %}
                    <a href="https://github.com/django-oscar/django-oscar-paypal">
                    {% if anon_checkout_allowed or request.user.is_authenticated %}
                        <a href="{% url 'paypal-redirect' %}">
                            <img src="https://www.paypal.com/en_US/i/btn/btn_xpressCheckout.gif" align="left" style="margin-right:7px;">
                        </a>
                    {% endif %}
                {% elif request.session.payment_method == "stripe"  %}
                    <a href="{% url 'gateway_redirect' %}">Stripe</a>
                {% elif request.session.payment_method == "paypal"  %}
                    <script src="https://www.paypal.com/sdk/js?client-id={Client ID}&currency=EUR"></script>

                    <div id="paypal-button-container"></div>

                    <script>

                        const basketId = "{{ basket.id }}";


                        paypal.Buttons({

                            async createOrder() {
                                const res = await fetch("/create-order/", {
                                    method: "POST",
                                    headers: {
                                        "X-CSRFToken": "{{ csrf_token }}"
                                    }
                                });

                                const data = await res.json();
                                return data.orderID;
                            },


                            async onApprove(data) {
                                const res = await fetch("/capture-order/", {
                                    method: "POST",
                                    headers: {
                                        "X-CSRFToken": "{{ csrf_token }}",
                                        "Content-Type": "application/json"
                                    },
                                    body: JSON.stringify({
                                        orderID: data.orderID,
                                        basketId: basketId
                                    })
                                });

                                const details = await res.json();

                                if (!res.ok) {
                                    console.error(details);
                                    alert("Error capturing the payment.");
                                    return;
                                }

                                window.location.href = "/success/" + basketId + "/";
                            },

                            async onCancel(data) {                

                                window.location.href = "/cancel/" + basketId + "/";
                            }

                        }).render("#paypal-button-container");

                    </script>
                {% endif %}
            </div>
        </div>
    </div>

{% endblock place_order %}