Buy credit for your own balance with your saved card [write]

Charges the caller's saved card the credit plus the card processing fee and adds the credit to their balance. amount is 1000, 2000, 5000 or 10000 cents. request_id makes a repeat of the same request charge once (200 instead of 201). Auto top-up is not changed. 409 card_required without a saved card (POST /billing/card-setup adds one), 402 card_declined, 503 payments_unavailable where card payments are not set up. A workspace key or a restricted credential cannot buy credit.