billing: Link customer stripe ID for manual fixed-price plan invoices.

For cases where a customer is created in Stripe first for the manual
invoice, we want to connect that customer information to our Customer
object in the database. That way the customer can access the billing
page once the invoice is paid.

corporate/lib/stripe.py

@@ -1496,18 +1496,42 @@ class BillingSession(ABC):
             current_plan.save(update_fields=["status", "next_invoice_date"])
             return f"Fixed price {required_plan_tier_name} plan scheduled to start on {current_plan.end_date.date()}."
 
+        # TODO: Use normal 'pay by invoice' flow for fixed-price plan offers,
+        # which requires handling automated license management for these plan
+        # offers via that flow.
         if sent_invoice_id is not None:
             sent_invoice_id = sent_invoice_id.strip()
-            # Verify 'sent_invoice_id' before storing in database.
+            # Verify 'sent_invoice_id' and 'stripe_customer_id' before
+            # storing in database.
             try:
                 invoice = stripe.Invoice.retrieve(sent_invoice_id)
                 if invoice.status != "open":
                     raise SupportRequestError(
                         "Invoice status should be open. Please verify sent_invoice_id."
                     )
+                invoice_customer_id = invoice.customer
+                if not invoice_customer_id:  # nocoverage
+                    raise SupportRequestError(
+                        "Invoice missing Stripe customer ID. Please review invoice."
+                    )
+                if customer.stripe_customer_id and customer.stripe_customer_id != str(
+                    invoice_customer_id
+                ):  # nocoverage
+                    raise SupportRequestError(
+                        "Invoice Stripe customer ID does not match. Please attach invoice to correct customer in Stripe."
+                    )
             except Exception as e:
                 raise SupportRequestError(str(e))
 
+            if customer.stripe_customer_id is None:
+                # Note this is an exception to our normal support panel actions,
+                # which do not set any stripe billing information. Since these
+                # invoices are manually created first in stripe, it's important
+                # for our billing page to have our Customer object correctly
+                # linked to the customer in stripe.
+                customer.stripe_customer_id = str(invoice_customer_id)
+                customer.save(update_fields=["stripe_customer_id"])
+
             fixed_price_plan_params["sent_invoice_id"] = sent_invoice_id
             Invoice.objects.create(
                 customer=customer,