Will the user be notified when a credit card fails on an Auto Top-up?

Yes. Tall Bob notifies the user when their credit card fails

When a user's credit card fails, can they still send messages?

Tall Bob sends the messages up to a negative (-ve) amount equal to the amount in the auto top-up

If a user can't send messages, will the messages be sent when the user top-ups the balance?

Unfortunately, if the message sending has failed due to insufficient credits, they would have to be resent to ensure message delivery

What would happen when the user sends messages via the Tall Bob API?

Tall Bob payment options for API send work similarly to the platform send.