Add a card from Billing. The default card is what auto-recharge uses when
the remaining credit would not cover the current burn.
A working saved card silences the low-credit warning emails: the falling
balance is the trigger that charges the card, not a problem you have to act
on. You get the payment receipt when the charge succeeds. Warnings resume
only after a charge fails.
You cannot remove the last working payment method. A failed card is not
cover, so it does not unlock removing the working one. Removing the default
promotes another active method if one exists.