Yikes, we don't want you having to travel to a store to do this. Are you able to pay over the phone with the same card? Also, have you tried through the app to see if it works?
I'm sorry this error is causing you to have to go out of your way to make a payment towards your account. You always have the option of dialing *ADD (*233) while we're getting our site back up and running. Keep us posted if you're still having issues.