Received a "renewal failed" notice and went in to update the expired card that had been in the account. FastSpring would not allow the new (valid) card information to be successfully entered. Nor would it allow a different (valid) card to be successfully entered. I rather expected to be able to update my information and renew my subscription.
Not sure you can. I'm certainly not going to leave my payment information here.
Closing this since this is an issue to resolve by sending an email to contact@worldanvil.