If an email invoice is undeliverable, will it fail to be paid?
card_id has been registered.
If the card_id is valid, then we will try to charge the card without emailing the invoice. If the card fails, and the email is also undeliverable, then yes, it would remain unpaid and you would need to manually update it (change the email/card), else we will simply keep retrying each invoice period and it will keep failing.