Web site won't let me add a payment method

I can't add a new credit card to my account when using the web site from a desktop computer with Firefox for Windows (v62.0).

After logging in, I select Profile, then click Edit next to Manage Payment Options.  I briefly see a Visa logo and Edit/Remove links for my current credit card,  however the page immediately refreshes and takes me back to the main account overview page.

So, from the overview page, I tried selecting the REFILL ACCOUNT button in the My Account Status section.  That takes me to a page with options to Fund Your Account.  There I clicked Use A Credit Card.  Same thing.  I briefly see a page with some credit card information and it immediately refreshes and takes me back to the account overview page.

Finally, I clicked on View/Edit for the Auto-Pay Settings on the account overview page.  Same thing... auto-refresh back to the beginning.

It seems that this is happening because I'm a pre-paid customer.

I'm at a loss to understand why they would prevent pre-paid users from managing their payment options online, forcing us to call Customer Service.

I tried starting a chat session from the web site, and the support rep said "sorry, I can't help you with pre-paid account, you'll have to call them."

What a waste of my time.  Very frustrating, to say the least.