At the time of app installation, the add-ons are added by the marketplace for paid apps. Billing team won't be able to add the add-ons.
For online subscription accounts, when the customer adds their card on file, it will be charged & invoice will be generated spontaneously. In case of card declines, customers can try to change the credit card from Plans & Billing, and try again.
For offline subscription accounts, the invoice will be generated automatically with the existing payment terms.