I need an invoice
You can download your invoice from the Account & Billing tab of your profile. Just scroll down to find your Billing History and click Get invoice;
Note that if you put several items in a single order and pay for all of them at once you will not see the Get Invoice button for every subscription from the order in the Billing History table.
If you purchase the subscription in the Android or iOS app, you'll get a receipt email from Google Play or App Store.