Use billing email to create subscriptions

We can't use an email from GL.com because
it may differ from the billing email.
Instead we must use the email
received from the CustomersDot as a billing email.

Closes https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2168

ee/app/services/subscriptions/create_service.rb

@@ -13,10 +13,16 @@ module Subscriptions
 
     def execute
       response = client.create_customer(create_customer_params)
+
       return response unless response[:success]
 
-      token = response.with_indifferent_access[:data][:customer][:authentication_token]
-      client.create_subscription(create_subscription_params, current_user.email, token)
+      # We can't use an email from GL.com because it may differ from the billing email.
+      # Instead we use the email received from the CustomersDot as a billing email.
+      customer_data = response.with_indifferent_access[:data][:customer]
+      billing_email = customer_data[:email]
+      token = customer_data[:authentication_token]
+
+      client.create_subscription(create_subscription_params, billing_email, token)
     end
 
     private

ee/changelogs/unreleased/vs-use-billing-email-to-create-subscriptions.yml

+---
+title: Use billing email for purchasing subscriptions
+merge_request: 47105
+author:
+type: fixed

ee/spec/services/subscriptions/create_service_spec.rb

@@ -28,6 +28,7 @@ RSpec.describe Subscriptions::CreateService do
     }
   end
 
+  let_it_be(:customer_email) { 'first.last@gitlab.com' }
   let_it_be(:client) { Gitlab::SubscriptionPortal::Client }
   let_it_be(:create_service_params) { Gitlab::Json.parse(fixture_file('create_service_params.json', dir: 'ee')).deep_symbolize_keys }
 
@@ -44,11 +45,11 @@ RSpec.describe Subscriptions::CreateService do
 
     context 'when successfully creating a customer' do
       before do
-        allow(client).to receive(:create_customer).and_return(success: true, data: { success: true, 'customer' => { 'authentication_token' => 'token' } })
+        allow(client).to receive(:create_customer).and_return(success: true, data: { success: true, 'customer' => { 'authentication_token' => 'token', 'email' => customer_email } })
       end
 
       it 'creates a subscription with the returned authentication token' do
-        expect(client).to receive(:create_subscription).with(anything, user.email, 'token')
+        expect(client).to receive(:create_subscription).with(anything, customer_email, 'token')
         subject.execute
       end
 
@@ -75,7 +76,7 @@ RSpec.describe Subscriptions::CreateService do
 
     context 'passing the correct parameters to the client' do
       before do
-        allow(client).to receive(:create_customer).and_return(success: true, data: { success: true, customer: { authentication_token: 'token' } })
+        allow(client).to receive(:create_customer).and_return(success: true, data: { success: true, customer: { authentication_token: 'token', email: customer_email } })
         allow(client).to receive(:create_subscription).and_return(success: true, data: { success: true, subscription_id: 'xxx' })
       end
 
@@ -86,7 +87,7 @@ RSpec.describe Subscriptions::CreateService do
       end
 
       it 'passes the correct parameters for creating a subscription' do
-        expect(client).to receive(:create_subscription).with(create_service_params[:subscription], 'first.last@gitlab.com', 'token')
+        expect(client).to receive(:create_subscription).with(create_service_params[:subscription], customer_email, 'token')
 
         subject.execute
       end