Originally Posted by thunderdeacon

I have experienced this issue multiple times. Based on my experience, you will not be able to pay for this online. You can't even use it for a new ticket, as it will give an error message. If the travel credit completely covers the price of the new ticket, you can use the chat function on the app or use Twitter. If you will have to make an additional payment you will have to call it in, as that is the only place they can accept credit card information.