Click the card in the User Cards list — the one you click becomes the default (shown with a red "Default" ribbon) for future charges on that Stripe account. In the current code this specific action doesn't re-check the Edit Users permission the way adding or deleting a card does — it only checks that you're logged into the admin panel — but in practice that's not an opening, since Edit Users is already required just to open the customer's profile in the first place.