gistfile1.txt

# models.py 

def subscribe(self, plan, quantity=1, trial_days=None,
              charge_immediately=True, prorate=djstripe_settings.PRORATION_POLICY, discount_code=None):
    cu = self.stripe_customer
    """
    Trial_days corresponds to the value specified by the selected plan
    for the key trial_period_days.
    """
    if ("trial_period_days" in djstripe_settings.PAYMENTS_PLANS[plan]):
        trial_days = djstripe_settings.PAYMENTS_PLANS[plan]["trial_period_days"]

    if trial_days:
        resp = cu.update_subscription(
            plan=djstripe_settings.PAYMENTS_PLANS[plan]["stripe_plan_id"],
            trial_end=timezone.now() + datetime.timedelta(days=trial_days),
            prorate=prorate,
            quantity=quantity,
            coupon=coupon
        )
    else:
        resp = cu.update_subscription(
            plan=djstripe_settings.PAYMENTS_PLANS[plan]["stripe_plan_id"],
            prorate=prorate,
            quantity=quantity,
            coupon=coupon
        )
    self.sync_current_subscription()
    if charge_immediately:
        self.send_invoice()
    subscription_made.send(sender=self, plan=plan, stripe_response=resp)
    
# forms.py

class PlanForm(forms.Form):
    plan = forms.ChoiceField(choices=PLAN_CHOICES)
    coupon = forms.CharField(max_length=30, required=False)
    
# views.py 

class SubscribeFormView(LoginRequiredMixin, FormValidMessageMixin, SubscriptionMixin, FormView):
    # TODO - needs tests

    form_class = PlanForm
    template_name = "djstripe/subscribe_form.html"
    success_url = reverse_lazy("djstripe:account")
    form_valid_message = "Congratulations! You've successfully subscribed to Zana Edge. We'll be in touch shortly via email."

    def post(self, request, *args, **kwargs):
        """
        Handles POST requests, instantiating a form instance with the passed
        POST variables and then checked for validity.
        """
        form_class = self.get_form_class()
        form = self.get_form(form_class)
        if form.is_valid():
            try:
                customer, created = Customer.get_or_create(
                    subscriber=subscriber_request_callback(self.request))
                customer.update_card(self.request.POST.get("stripe_token"))
                customer.subscribe(form.cleaned_data["plan"], coupon=coupon)
            except stripe.StripeError as e:
                # add form error here
                self.error = e.args[0]
                return self.form_invalid(form)
            # redirect to confirmation page
            return self.form_valid(form)
        else:
            return self.form_invalid(form)

I changed "discount_code" to "coupon", I just didn't show that part of the code in my latest comment. I moved the docstring. I'm not sure what you mean in your fourth point. How do I extend? Do I just make a view in my own project that has the same title like I've done with the subscribe_form template?

yourviews.py

from djstripe.views import SubscribeFormView

class MySubscribeFormView(SubscribeFormView):
    form_valid_message = "Congratulations! You've successfully subscribed to Zana Edge. We'll be in touch shortly via email."

yoururls.py

from .views import MySubscribeFormView

url("^subscribe/$", MySubscribeFormView.as_view(), name="mysubscribe")

I'm not sure if setting the name to djstripe:subscribe and adding it above the djstripe url import will work. Give it a try though - if it doesn't work, you just need to change the action on the subscribe form here to {% url 'mysubscribe' %} either by overriding the entire template or setting it via javascript. If you've already customized that template (djstripe/subscribe_form.html) and haven't overridden it outside the package, I can help with that too.